[BUG] [flytekit][mypy] Fix class name is not defined

[pingsutw]

Describe the bug
Related to #1030
Fix mypy errors like "Name "Builder" is not defined"

flytekit/core/context_manager.py:265: error: Name "TaskResolverMixin" is not defined
flytekit/core/context_manager.py:519: error: Name "Builder" is not defined
flytekit/core/context_manager.py:542: error: Name "Builder" is not defined
flytekit/core/context_manager.py:546: error: Name "Builder" is not defined
flytekit/core/context_manager.py:550: error: Name "Builder" is not defined
flytekit/core/context_manager.py:553: error: Name "Builder" is not defined
flytekit/core/context_manager.py:557: error: Name "Builder" is not defined
flytekit/types/file/file.py:353: error: Name "literal_type.blob.format" is not defined
flytekit/types/directory/types.py:293: error: Name "literal_type.blob.format" is not defined
tests/flytekit/unit/core/test_flyte_file.py:35: error: Name "txt" is not defined
tests/flytekit/unit/core/test_flyte_file.py:42: error: Name "txt" is not defined

Expected behavior
Should pass the mypy checks.

[Optional] Additional context
To Reproduce
Steps to reproduce the behavior:

1. make test | grep "not defined"

Screenshots
If applicable, add screenshots to help explain your problem.

[mayitbeegh]

Fixed in flyteorg/flytekit#645