Add module reader to enable module dependent variables

[Nepomuceno]

This Pr it is part of a larger PR it order to add variable auto completion for modules in the terraform language server. hashicorp/terraform-ls#93
a complementary implementation in the language server side can be found at
hashicorp/terraform-ls#551

[radeksimko]

Except for my in-line comments (most of which are minor) this looks good!

Can you also add a new test to schema/schema_merge_test.go which exercises the new codepath? i.e. has a simple mock implementation of the ModuleReader interface (can be literally hard-coded return values) and checks that the module block has the expected schema?

Thanks!

[radeksimko]

LGTM, I think this is ready for merging, but before doing so I'd like to also see hashicorp/terraform-ls#551 modified to leverage this PR - so we're sure that everything fits well together.

[Nepomuceno]

LGTM, I think this is ready for merging, but before doing so I'd like to also see hashicorp/terraform-ls#551 modified to leverage this PR - so we're sure that everything fits well together.

Working on that now just wanted to make sure this interface would not change.