terraform_rules: Add workaround for skipping child modules inspection

[wata727]

Fixes #878

This PR adds a workaround to fix false positives in terraform rules.

Module inspection emits an issue in a module argument if an expression contains the module argument. However, this works as expected for rules such as aws_instance_invalid_type, but not for rules such as terraform_deprecated_interpolation, which has no cause in module arguments.

In order to avoid such false positives, first, in the rules of terraform, analysis other than the root module is explicitly skipped. This is not a good idea, but the goal is first to avoid false positives.

[bendrucker]

I would change "This rule does not work for child modules" to "This rule does not evaluate child modules." Since they themselves do work, they're just electing not to enforce themselves on certain modules.

This definitely works in the near term to take care of the regression. terraform_workspace_remote raises an interesting counter-case. Including terraform.workspace in a child module has the same unexpected consequences in Terraform Cloud as using it in the root module would. Ideally that rule should be able to emit errors from child modules and those errors should not be associated with a module variable.

#769 might have the same need. Addressing it is probably a pretty far-ranging change—this is good for now.