feat(cmake): pkgconf-pypi entrypoint

[virtuald]

• This allows pkgconf --cflags pybind11 and similar commands to work as expected if pkgconf is installed from pypi

Description

pkgconf is used by build tools such as meson to find pkg-config files. In a virtualenv, the system pkgconf cannot find pybind11 installed in the virtualenv (though meson has other ways to do this).

https://github.com/pypackaging-native/pkgconf-pypi packages pkgconf into a wheel, and uses entrypoints to extend the PKG_CONFIG_PATH so that .pc files can be found in the virtualenv (docs).

pybind11 already distributes a .pc file in the wheel. This PR adds an entrypoint so that it's usable with pkgconf-pypi.

Suggested changelog entry:

Added support for finding pybind11 using pkgconf distributed on pypi

[FFY00]

FYI, directories don't need a __init__.py to be importable. They are called namespace packages. They're slightly cursed, but they're a thing 😅

https://packaging.python.org/en/latest/guides/packaging-namespace-packages/#native-namespace-packages

[virtuald]

I'm pretty sure that I tried it and it didn't work. I believe namespace packages are only a thing at the top level package, not for subpackages? ... I don't remember why it didn't work, but it didn't work.

[FFY00]

No, they should work for any. Perhaps the pkgconf-pypi code is broken, I was looking at it today, we'll see if I can push my changes and tests tomorrow.

[henryiii]

I think I've run into issues with importlib.resources on namespace packages, could be related?

[FFY00]

That would likely be because we only added support for namespace packages in importlib.resources in 3.10 (python/importlib_resources#196).

It shouldn't be affecting this since we aren't using importlib.resources. We did a while ago, but we were using the backport for < 3.10 (it was dropped in pypackaging-native/pkgconf-pypi@bb52fe1).

[henryiii]

I've had to add workarounds in #5598 to generate the __init__.py files, would be nice if I could get rid of them. :)