[red-knot] Use vendored typeshed stubs for stdlib module resolution

[AlexWaygood]

Summary

If the user does not pass a --custom-typeshed-dir flag via the CLI or via a configuration flag, fall back to using the VendoredFileSystem to resolve standard-library modules in the module resolver. Work towards #11653.

(This PR is stacked on top of #12215)

Test Plan

I added a test to resolver.rs that demonstrates that vendored files now work

[github-actions[bot]]

ruff-ecosystem results

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Linter (preview)

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