fix unknown imports being incorrectly highlighted as a variable

[DetachHead]

fixes #414

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

sympy (https://github.com/sympy/sympy)
-     Type of "P" is "(condition: Unknown, given_condition: Unknown | None = None, numsamples: Unknown | None = None, evaluate: bool = True, **kwargs: Unknown) -> (Unknown | Any | BernoulliDistribution | Probability | Self@Expr | tuple[Unknown, ...] | Sum | Order | Piecewise | Basic | Equality | Lambda | Relational | Unequality | int | None)" (reportUnknownVariableType)
+     Type of "P" is "(condition: Unknown, given_condition: Unknown | None = None, numsamples: Unknown | None = None, evaluate: bool = True, **kwargs: Unknown) -> (Unknown | Any | BernoulliDistribution | Probability | Self@Expr | tuple[Unknown, ...] | Sum | Order | Piecewise | Basic | Equality | Lambda | int | None)" (reportUnknownVariableType)
-     Type of "E" is "(expr: Unknown, condition: Unknown | None = None, numsamples: Unknown | None = None, evaluate: bool = True, **kwargs: Unknown) -> (Unknown | Basic | Expectation | tuple[Unknown, ...] | Sum | Self@Expr | Order | Any | Piecewise | Equality | Relational | Unequality | Integral | ExpectationMatrix | Literal[0] | None)" (reportUnknownVariableType)
+     Type of "E" is "(expr: Unknown, condition: Unknown | None = None, numsamples: Unknown | None = None, evaluate: bool = True, **kwargs: Unknown) -> (Unknown | Basic | Expectation | tuple[Unknown, ...] | Sum | Self@Expr | Order | Any | Piecewise | Equality | Integral | ExpectationMatrix | Literal[0] | None)" (reportUnknownVariableType)
-     Type of "H" is "(expr: Unknown, condition: Unknown | None = None, **kwargs: Unknown) -> (int | Expr | Basic | Expectation | Unknown | tuple[Unknown, ...] | Sum | Self@Expr | Order | Any | Piecewise | Equality | Relational | Unequality | Integral | ExpectationMatrix | None)" (reportUnknownVariableType)
+     Type of "H" is "(expr: Unknown, condition: Unknown | None = None, **kwargs: Unknown) -> (int | Expr | Basic | Expectation | Unknown | tuple[Unknown, ...] | Sum | Self@Expr | Order | Any | Piecewise | Equality | Integral | ExpectationMatrix | None)" (reportUnknownVariableType)
-     Type of "covariance" is "(X: Unknown, Y: Unknown, condition: Unknown | None = None, **kwargs: Unknown) -> (CrossCovarianceMatrix | Covariance | Unknown | Basic | Expectation | tuple[Unknown, ...] | Sum | Self@Expr | Order | Any | Piecewise | Equality | Relational | Unequality | Integral | ExpectationMatrix | Literal[0] | None)" (reportUnknownVariableType)
+     Type of "covariance" is "(X: Unknown, Y: Unknown, condition: Unknown | None = None, **kwargs: Unknown) -> (CrossCovarianceMatrix | Covariance | Unknown | Basic | Expectation | tuple[Unknown, ...] | Sum | Self@Expr | Order | Any | Piecewise | Equality | Integral | ExpectationMatrix | Literal[0] | None)" (reportUnknownVariableType)
-     Type of "entropy" is "(expr: Unknown, condition: Unknown | None = None, **kwargs: Unknown) -> (int | Expr | Basic | Expectation | Unknown | tuple[Unknown, ...] | Sum | Self@Expr | Order | Any | Piecewise | Equality | Relational | Unequality | Integral | ExpectationMatrix | None)" (reportUnknownVariableType)
+     Type of "entropy" is "(expr: Unknown, condition: Unknown | None = None, **kwargs: Unknown) -> (int | Expr | Basic | Expectation | Unknown | tuple[Unknown, ...] | Sum | Self@Expr | Order | Any | Piecewise | Equality | Integral | ExpectationMatrix | None)" (reportUnknownVariableType)
-     Type of "factorial_moment" is "(X: Unknown, n: Unknown, condition: Unknown | None = None, **kwargs: Unknown) -> (Expr | Basic | Expectation | Unknown | tuple[Unknown, ...] | Sum | Self@Expr | Order | Any | Piecewise | Equality | Relational | Unequality | Integral | ExpectationMatrix | Literal[0] | None)" (reportUnknownVariableType)
+     Type of "factorial_moment" is "(X: Unknown, n: Unknown, condition: Unknown | None = None, **kwargs: Unknown) -> (Expr | Basic | Expectation | Unknown | tuple[Unknown, ...] | Sum | Self@Expr | Order | Any | Piecewise | Equality | Integral | ExpectationMatrix | Literal[0] | None)" (reportUnknownVariableType)
-   /tmp/mypy_primer/projects/sympy/sympy/stats/compound_rv.py:107:9 - warning: Return type, "tuple[Unknown, ...] | Unknown | Sum | Self@Expr | Order | Any | Piecewise | Basic | Equality | Relational | Unequality | Integral | Literal[0] | None", is partially unknown (reportUnknownParameterType)
+   /tmp/mypy_primer/projects/sympy/sympy/stats/compound_rv.py:107:9 - warning: Return type, "tuple[Unknown, ...] | Unknown | Sum | Self@Expr | Order | Any | Piecewise | Basic | Equality | Integral | Literal[0] | None", is partially unknown (reportUnknownParameterType)
-     Type of "compute_expectation" is "(expr: Unknown, rvs: Unknown | None = None, **kwargs: Unknown) -> (tuple[Unknown, ...] | Unknown | Sum | Self@Expr | Order | Any | Piecewise | Basic | Equality | Relational | Unequality | Literal[0] | None)" (reportUnknownMemberType)
+     Type of "compute_expectation" is "(expr: Unknown, rvs: Unknown | None = None, **kwargs: Unknown) -> (tuple[Unknown, ...] | Unknown | Sum | Self@Expr | Order | Any | Piecewise | Basic | Equality | Literal[0] | None)" (reportUnknownMemberType)
-   /tmp/mypy_primer/projects/sympy/sympy/stats/compound_rv.py:113:20 - warning: Return type, "tuple[Unknown, ...] | Unknown | Sum | Self@Expr | Order | Any | Piecewise | Basic | Equality | Relational | Unequality | Literal[0] | None", is partially unknown (reportUnknownVariableType)
+   /tmp/mypy_primer/projects/sympy/sympy/stats/compound_rv.py:113:20 - warning: Return type, "tuple[Unknown, ...] | Unknown | Sum | Self@Expr | Order | Any | Piecewise | Basic | Equality | Literal[0] | None", is partially unknown (reportUnknownVariableType)
-     Type of "compute_expectation" is "((expr: Unknown, rvs: Unknown | None = None, evaluate: bool = False, **kwargs: Unknown) -> (Unknown | Equality | Relational | Unequality | Integral)) | ((expr: Unknown, rvs: Unknown | None = None, evaluate: bool = True, **kwargs: Unknown) -> (Unknown | Equality | Relational | Unequality | Sum))" (reportUnknownMemberType)
+     Type of "compute_expectation" is "((expr: Unknown, rvs: Unknown | None = None, evaluate: bool = False, **kwargs: Unknown) -> (Unknown | Equality | Integral)) | ((expr: Unknown, rvs: Unknown | None = None, evaluate: bool = True, **kwargs: Unknown) -> (Unknown | Equality | Sum))" (reportUnknownMemberType)
-   /tmp/mypy_primer/projects/sympy/sympy/stats/compound_rv.py:114:16 - warning: Return type, "Unknown | Equality | Relational | Unequality | Integral | Sum", is partially unknown (reportUnknownVariableType)
+   /tmp/mypy_primer/projects/sympy/sympy/stats/compound_rv.py:114:16 - warning: Return type, "Unknown | Equality | Integral | Sum", is partially unknown (reportUnknownVariableType)
-   /tmp/mypy_primer/projects/sympy/sympy/stats/compound_rv.py:116:9 - warning: Return type, "Unknown | Probability | Self@Expr | tuple[Unknown, ...] | Sum | Order | Any | Piecewise | Basic | Equality | Relational | Unequality | int", is partially unknown (reportUnknownParameterType)
+   /tmp/mypy_primer/projects/sympy/sympy/stats/compound_rv.py:116:9 - warning: Return type, "Unknown | Probability | Self@Expr | tuple[Unknown, ...] | Sum | Order | Any | Piecewise | Basic | Equality | int", is partially unknown (reportUnknownParameterType)
-     Type of "probability" is "((condition: Unknown, **kwargs: Unknown) -> Unknown) | ((condition: Unknown) -> (Unknown | Probability | Self@Expr | tuple[Unknown, ...] | Sum | Order | Any | Piecewise | Basic | Equality | Relational | Unequality | int)) | ((condition: Unknown) -> (Unknown | Piecewise | Literal[0]))" (reportUnknownMemberType)
+     Type of "probability" is "((condition: Unknown, **kwargs: Unknown) -> Unknown) | ((condition: Unknown) -> (Unknown | Probability | Self@Expr | tuple[Unknown, ...] | Sum | Order | Any | Piecewise | Basic | Equality | int)) | ((condition: Unknown) -> (Unknown | Piecewise | Literal[0]))" (reportUnknownMemberType)
-   /tmp/mypy_primer/projects/sympy/sympy/stats/compound_rv.py:119:16 - warning: Return type, "Unknown | Probability | Self@Expr | tuple[Unknown, ...] | Sum | Order | Any | Piecewise | Basic | Equality | Relational | Unequality | int", is partially unknown (reportUnknownVariableType)
+   /tmp/mypy_primer/projects/sympy/sympy/stats/compound_rv.py:119:16 - warning: Return type, "Unknown | Probability | Self@Expr | tuple[Unknown, ...] | Sum | Order | Any | Piecewise | Basic | Equality | int", is partially unknown (reportUnknownVariableType)
-     Type of "expr" is "tuple[Unknown, ...] | Unknown | Sum | Self@Expr | Order | Any | Piecewise | Basic | Equality | Relational | Unequality | Integral | None" (reportUnknownVariableType)
+     Type of "expr" is "tuple[Unknown, ...] | Unknown | Sum | Self@Expr | Order | Any | Piecewise | Basic | Equality | Integral | None" (reportUnknownVariableType)
-     Type of "_marginalise" is "(expr: Unknown, rv: Unknown, evaluate: Unknown) -> (tuple[Unknown, ...] | Unknown | Sum | Self@Expr | Order | Any | Piecewise | Basic | Equality | Relational | Unequality | Integral | None)" (reportUnknownMemberType)
+     Type of "_marginalise" is "(expr: Unknown, rv: Unknown, evaluate: Unknown) -> (tuple[Unknown, ...] | Unknown | Sum | Self@Expr | Order | Any | Piecewise | Basic | Equality | Integral | None)" (reportUnknownMemberType)
-   /tmp/mypy_primer/projects/sympy/sympy/stats/compound_rv.py:197:9 - warning: Return type, "tuple[Unknown, ...] | Unknown | Sum | Self@Expr | Order | Any | Piecewise | Basic | Equality | Relational | Unequality | Integral | None", is partially unknown (reportUnknownParameterType)
+   /tmp/mypy_primer/projects/sympy/sympy/stats/compound_rv.py:197:9 - warning: Return type, "tuple[Unknown, ...] | Unknown | Sum | Self@Expr | Order | Any | Piecewise | Basic | Equality | Integral | None", is partially unknown (reportUnknownParameterType)
-     Type of "expr" is "Equality | Unknown | Relational | Unequality | Sum" (reportUnknownVariableType)

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