vendored_typeshed_versions should use db.vendored

[MichaReiser]

Summary

This fixes a bug in the module resolver where it always defaulted to the versions file
bundled with Red Knot rather than using the vendored file system instance returned by db.vendored.

Not using db.vendored can lead to inconsistencies, as seen in the ruff module graph resolver where
the module resolver loads the versions file from the static vendored file system but resolves the modules from the empty vendored file system set on the ModuleGraphDb.

Now, this also shows that Ruff needs the vendored stubs. At least, the VERSIONS file because Red Knot verifies if the stub directory is valid when constructing the ProgramSettings to prevent the situation where a user runs red_knot fix with a broken custom typeshed dir.

Test Plan

cargo test

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[MichaReiser]

I think it's fine to defer validating the vendored typeshed versions. They should be valid and whether we panic here or when trying to resolve the first module doesn't make a real difference. It results in a panic.

The only downside is that getting the versions is now a salsa lookup rather than a static field access. Let's see if this hurts performance in a meaningful way

[MichaReiser]

Okay, there's a 1% performance drop. That doesn't seem worth it.

I'm just gonna make vendored_typeshed_versions always parse the file without caching. This is an overall simplification and shouldn't hurt performance except in the case when we reconstruct the SearchPathSettings because the project configuration has changed.... but that's a very slow path anyway and parsing the VERSIONS file should be rather fast

[MichaReiser]

Not sure why I didn't use Cow for this... 🫤

[MichaReiser]

I tried to make the typeshed versions usage as lazy as possible but there are some module graph tests that still access typeshed for imports that fail to resolve. We would have to add support for no typeshed to the module resolver to make the vendored stubs truly optional.