terraform test: Keep modules in state if they are required by the external references

[liamcervante]

This PR updates the v1.7 branch such that it won't remove "empty" modules from state if they have external references pointing to their outputs.

This does potentially increase the size of the state files, but currently it will only affect the terraform test framework as the external references are only set by the testing framework. This has been fixed "properly" in main, so it also means that slight hack implemented here will only be present in the v1.7 branch. Overall, I think that makes the approach here acceptable given the temporary nature of the fix. I had attempted to port over the real fix in #34477, but it was rightly pointed out that it's a bit overkill and potentially risky so close to an upcoming release. Hopefully, this is an acceptable middle ground.

Fixes #34476

Target Release

1.7.0-rc2

Draft CHANGELOG entry

BUG FIXES

• terraform test: Fix bug where outputs in "empty" modules were not available to the assertions from Terraform test files.

[apparentlymart]

This looks good to me!

It's encouraging that the new logic here is limited to only one function (nodeCloseModule.Execute) and that the rest of this is really just giving that function the data it needs to do that work.

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

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