Add validation logic for references to outputs from prior modules

[tpdownes]

This PR adds the validation logic for references to outputs from modules in earlier deployment groups. It does not yet add full support for these references; this will require further PRs to implement. However, it puts into place the validation logic such that these "intergroup references" are rejected for cases we never intend to support. For example, intergroup references are rejected when:

• the reference does not include an explicit group ID
• they refer to a later group

There is an implicit change here that allows the user to use references with explicit groups even in the "intragroup reference" case. The behavior of these references is identical to the status quo (module_id.output_name). I believe it would be more complicated to enforce and communicate "implicitness" requirements for intragroup references, so it is better to allow them.

With this PR, unit test coverage for the package improves from 84.9% to 85.3%.

Submission Checklist

• Have you installed and run this change against pre-commit? (pre-commit install)
• Are all tests passing? (make tests)
• Have you written unit tests to cover this change?
• Is unit test coverage still above 80%?
• Have you updated all applicable documentation?
• Have you followed the guidelines in our Contributing document?

[heyealex]

Thanks for the updates! LGTM