terraform test: move override evaluation into expander

[liamcervante]

This PR makes use of the new expander logic to handle the terraform test module overrides directly where the modules are expanded instead of later within the group as it was doing previously.

Previously, we just generally let the graph expansion happen and then looked for nodes within overridden modules as the graph executed. We'd then skip nodes within overridden modules to ensure they didn't execute, and then sideload values into the output nodes of the overridden modules.

This was actually broken for the expand nodes, as they aren't keyed by a module instance address which means it is impossible for us to tell during graph execution that they should be skipped. This meant that the expansion was still attempted for anything within an overridden module. If that expansion then required a value from something else in the module such as a local variable, the expansion failed with a crash. This is because the local variable execution was skipped (as it was in an overridden module), while the expansion was still attempted by the expansion nodes are not keyed by module instances.

Now, we pass the override information into the expander. When it is asked for module instances, it excludes instances that have been overridden. This means that concrete execute nodes for overridden modules are not even created as a result of that expansion essentially not even existing.

For output nodes, we actually do include the overridden module instances and we implement the logic for checking if the module instance has been overridden directly within the output node. This means the output nodes can load the values from the override, instead of attempting to look at any references which would then fail since the other nodes in the module were not executed.

The change count here is smaller than it looks. I changed some wide-ranging function arguments, which has led to a lot of diffs that simply set false or nil for the includeOverrides and overrides arguments within the expander and the "get module instances" functions.

Fixes #35019

Target Release

1.8.2

Draft CHANGELOG entry

NEW FEATURES | UPGRADE NOTES | ENHANCEMENTS | BUG FIXES | EXPERIMENTS

• terraform test: Prevent crash when referencing local variables within overridden modules.

[jbardin]

Not sure I understand that last sentence

[liamcervante]

Clarified!

[jbardin]

This looks like a lot, but from what I can tell this is well contained within the testing functionality, so should be OK to backport.

[github-actions]

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