Make py_wheel warn if BUILD (or other special bazel files) are included

We've seen there are wheels that have special bazel build-time artifacts in them, which then interfere with the pip integration. While we should remove these upon consuming a wheel to prevent the problem, the wheels also shouldn't have those files in them to begin with.

Something I wonder is _how_ those files are ending up in wheels? My guess is just naive `glob("**")` somewhere. We can't control other tools to create wheels, but at the least, we can change py_wheel to error or fail if such special files occur.

Hm, I wonder if we could query pypi to see how often the metadata `Generator` value indicates our py_wheel was used.

Special files:

* BUILD
* BUILD.bazel
* WORKSPACE
* WORKSPACE.bazel
* REPO.bazel
* MODULE.bazel
* Auto-generated `__init__.py` files

I include auto-generated init files because I've seen a few wheels (nvidia-something) that had empty `__init__.py` files in places they shouldn't be or made no sense. (they use namespace packages, so such empty init files break things).

Start by tracing the py_wheel entry point to determine how wheel contents are assembled and where existing validation could run. Clarify whether the expected behavior is a warning or failure, then cover the listed Bazel files and inappropriate auto-generated __init__.py files; done means the intended cases are detected without disrupting valid wheels.

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