Summary

The implicit namespace package rule currently fails to detect cases like the following:

foo/
├── __init__.py
└── bar/
    └── baz/
        └── __init__.py

The problem is that we detect a root at foo, and then an independent root at baz. We would detect that bar is an implicit namespace package, but it doesn't contain any files! So we never check it, and have no place to raise the diagnostic.

This PR adds detection for these kinds of nested packages, and augments the INP rule to flag the __init__.py file above with a specialized message. As a side effect, I've introduced a dedicated PackageRoot struct which we can pass around in lieu of Yet Another Path.

For now, I'm only enabling this in preview (and the approach doesn't affect any other rules). It's a bug fix, but it may end up expanding the rule.

Closes #13519.