Fix module visibility check

[jjcnn]

Description

Fixes #6673.

The module visibility check has been broken for some time.

In some cases, like the one described in #6673, this has resulted in modules being mistakenly regarded as inaccessible, even though they ought to be accessible.

In other cases, private modules were mistakenly regarded as accessible. For examples see the updated tests in the PR.

The check has now been fixed, and all tests have been updated accordingly.

This bugfix can be regarded as a breaking change, because some modules that were deemed accessible before may now be deemed inaccessible and will require the addition of a pub keyword in front of the module declaration.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[codspeed-hq]

CodSpeed Performance Report

Merging #6685 will not alter performance

_{Comparing jjcnn/fix_module_visibility_in_tests (3d95afb) with master (ccd8692)}

Summary

✅ 22 untouched benchmarks

[tritao]

This works, but to be clear what I had suggested originally was to make common_ancestor_count a local function inside check_module_privacy, unless you think it's going to be useful and called by something else?

[jjcnn]

Now that I've managed to reduce it to a single line of code, I'm inclined inline it again, and not have it as a separate function.

I doubt it needs to be used elsewhere.