## Summary
The implicit namespace package rule currently fails to detect cases like
the following:
```text
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 https://github.com/astral-sh/ruff/issues/13519.
c7d48e10e6