feat: add entrypoint for cmake modules dir

[henryiii]

Description

This can be used by tools (like scikit-build-core) to access the cmake modules directory. Suggested use:

module_dirs = importlib.metadata.entry_points(group="cmake.modules")
dirs = (importlib.resources.files(ep.load()) for ep in module_dirs)
paths_str = ";".join(str(p) for p in dirs)
cmake_arg = f"-DCMAKE_PREFIX_PATH={paths_str}"

Suggested changelog entry:

* Experimental support for ``cmake.modules`` entrypoint.