deprecated_index: restore evaluation of JSON expressions

[bendrucker]

• Fixes a false negative for legacy dot syntax usage in JSON files.
• Adds test coverage against valid JSON expressions.

Background

#90 fixed the problem of duplicate emitted issues for nested expressions, since WalkExpressions visits them recursively for HCL files. In doing so, it both fixed a bug related to good JSON files (#100) and created a regression in detecting dot syntax usage in JSON. There was no test coverage so I'm only now noticing this in hindsight.

Changes

This PR restores evaluation of JSON expressions, without reintroducing the bug described in #100. That issue seems to describe a case where hclsyntax.LexExpression is called on a raw range of source bytes:

tflint-ruleset-terraform/rules/terraform_deprecated_index.go

Line 63 in 70051da

tokens, diags := hclsyntax.LexExpression(bytes, filename, variable.SourceRange().Start)

When this range has escaped quotes, which are valid in the surrounding context, the raw content is not valid HCL. Given the previous implementation a more correct approach for JSON expressions would require parsing as JSON at some level to parse the escaped quotes:

var str
err := json.Unmarshal(bytes, &str)

Since WalkExpressions is not recursive for JSON, we still need to call Variables(). But from there we can then just pass each traversal to the existing checkLegacyTraversalIndex() function written for HCL, and it works!

[wata727]

Smart way! 👏