terraform_deprecated_index: Emit issues based on expression types

[wata727]

Fixes #84

The current deprecated index detection is based on lexical analysis. Since the rule lexes every expression walked by WalkExpressions, it has the problem of emitting multiple issues for the same expression when walking on nested expressions.

This PR switches deprecated index detection from lexical analysis-based to syntax analysis-based. Instead of lexing the walked expression, depending on the expression type, check if it has deprecated indexes.

hcl-parse tells us that the types of expressions we should be dealing with in this rule are ScopeTraversalExpr, RelativeTraversalExpr, and SplatExpr.

$ hcl-parse -e "list.0"
(*hclsyntax.ScopeTraversalExpr "list.0")

$ hcl-parse -e "map.*"
(*hclsyntax.SplatExpr
  (*hclsyntax.ScopeTraversalExpr "map")
  (*hclsyntax.AnonSymbolExpr)
)

$ hcl-parse -e "nested_list.*.0"
(*hclsyntax.SplatExpr
  (*hclsyntax.ScopeTraversalExpr "nested_list")
  (*hclsyntax.RelativeTraversalExpr ".0"
    (*hclsyntax.AnonSymbolExpr)
  )
)

This allows the rule to emit only one issue for nested expressions. Additionally, issues can be reported against a more precise range.

[bendrucker]

Nice, much better!