Provide diagnostics in tfvars files

[dbanck]

Problem Statement

With the current early validation implementation, we validate and publish diagnostics for all *.tf files inside a module. We can extend some of the validation rules to *.tfvars files.

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Part of hashicorp/vscode-terraform#720

Proposal

• Validate *.tfvars files
  • Flag up any unknown/undeclared variable names (without variable block)
  • Flag up any blocks
  • (nice to have) duplicate entries - i.e. the same variable declared more than once
    • Report & correct duplicate variable definitions across auto-loaded tfvars files #702
    • Stale diagnostics for *.json files #705
    • Publish diagnostics for non-autoloaded variable files (tfvars) #715
    • Publish diagnostics for hidden .*.tf files & non-autoloaded *.tfvars #970

Implementation Notes

Of all existing validators we'll probably only need two:

validator.UnexpectedAttribute{},
validator.UnexpectedBlock{},

validator.MissingRequiredAttribute{}, may look relevant but we have to keep in mind that variable values can be supplied from other places such as CLI flags, not only tfvars files, therefore we cannot expect all required variables to be declared in *.tfvars files.

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[radeksimko]

This was implemented as part of #1413 and the "nice to haves" can be followed in individual issues as linked above.

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