Add terraform_standard_module_structure rule

[bendrucker]

This PR creates a rule that enforces the Terraform standard module structure:

https://www.terraform.io/docs/modules/index.html#standard-module-structure

The rule itself ensures that main, variables, and outputs file are created. It also ensures that variable/output blocks are included in the corresponding file. File checks are skipped for JSON-only projects and block checks are skipped when the declaration was in a JSON file.

Outside of the rule implementation, I've also added logic to TestRunnerWithConfig that detects the directory so that rules can test configuration that is in a directory other than ..

[mveitas]

The standard module structure also enforces that a readme exist within the module (README or README.md). Thoughts on including this?

[bendrucker]

I did think about that but chose to skip it:

• Validating non-Terraform files would be new and possibly unexpected
• Readmes are optional for nested modules and TFLint cannot distinguish a nested module if it is called directly on it (cd modules/foo && tflint .)

[bendrucker]

Other related ideas here:

• Rule for community filename conventions for root modules (providers.tf, backend.tf)
• Config for required_version/required_providers rules allowing them to enforce the conventional filename (versions.tf)

[wata727]

👍