resource_missing_tags: Ignore non-existent provider aliases

[wata727]

Fixes #518

The aws_resource_missing_tags rule implicitly expects the provider declaration to be in the same module if a resource has a provider argument. However, In child modules, provider declarations are passed implicitly or explicitly from the root module. In this case, it is not possible to refer to the provider's default_tags in this module.

https://developer.hashicorp.com/terraform/language/modules/develop/providers

The solution to this problem is to parse the provider declaration passed from the root module in module inspection. However, the current module inspection design has many limitations that unfortunately cannot be solved in this way. This issue is tracked in terraform-linters/tflint-plugin-sdk#193

As a workaround, if the provider declaration is missing in the module, rather than returning an error, ignore the default tags. Conservatively, some default tags might apply, so you could skip the inspection, but there are probably many cases where it's more convenient to simply ignore the default tags.