Detect empty implicit namespace packages

[charliermarsh]

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.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

ℹ️ ecosystem check detected linter changes. (+2 -0 violations, +0 -0 fixes in 1 projects; 53 projects unchanged)

bokeh/bokeh (+2 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --output-format concise --preview --select ALL

+ tests/unit/bokeh/embed/ext_package_no_main/__init__.py:1:1: INP001 File `tests/unit/bokeh/embed/ext_package_no_main/__init__.py` declares a package, but is nested under an implicit namespace package. Add an `__init__.py` to `tests/unit/bokeh/embed`.
+ tests/unit/bokeh/embed/latex_label/__init__.py:1:1: INP001 File `tests/unit/bokeh/embed/latex_label/__init__.py` declares a package, but is nested under an implicit namespace package. Add an `__init__.py` to `tests/unit/bokeh/embed`.

Changes by rule (1 rules affected)

code	total	+ violation	- violation	+ fix	- fix
INP001	2	2	0	0	0

[MichaReiser]

Is it intentional that this file is commited?