astropy: Warnings about ignored packages

Description

With the switch from setup.cfg to pyproject.toml (#15247) the installation log now reports many warnings about ignored packages, e.g.:

check.warn(importable)
  /tmp/simon/pip-build-env-990sh2z5/overlay/lib/python3.11/site-packages/setuptools/command/build_py.py:204: _Warning: Package 'astropy.convolution.src' is absent from the `packages` configuration.
  !!

          ********************************************************************************
          ############################
          # Package would be ignored #
          ############################
          Python recognizes 'astropy.convolution.src' as an importable package[^1],
          but it is absent from setuptools' `packages` configuration.

          This leads to an ambiguous overall configuration. If you want to distribute this
          package, please make sure that 'astropy.convolution.src' is explicitly added
          to the `packages` configuration field.

          Alternatively, you can also rely on setuptools' discovery methods
          (for example by using `find_namespace_packages(...)`/`find_namespace:`
          instead of `find_packages(...)`/`find:`).

          You can read more about "package discovery" on setuptools documentation page:

          - https://setuptools.pypa.io/en/latest/userguide/package_discovery.html

          If you don't want 'astropy.convolution.src' to be distributed and are
          already explicitly excluding 'astropy.convolution.src' via
          `find_namespace_packages(...)/find_namespace` or `find_packages(...)/find`,
          you can try to use `exclude_package_data`, or `include-package-data=False` in
          combination with a more fine grained `package-data` configuration.

          You can read more about "package data files" on setuptools documentation page:

          - https://setuptools.pypa.io/en/latest/userguide/datafiles.html


          [^1]: For Python, any directory (with suitable naming) can be imported,
                even if it does not contain any `.py` files.
                On the other hand, currently there is no concept of package data
                directory, all directories are treated like packages.
          ********************************************************************************

and same for all data directories and other directories that don’t have an __init__.py file.

It’s related to https://setuptools.pypa.io/en/latest/userguide/package_discovery.html and the fact that setuptools now prefers the use of a src-layout (though I don’t know why switching from setup.cfg to pyproject.toml triggers this behavior). I had this issue with other packages and solved that by using a src-layout but it seems astropy is not ready for that yet. However we should find a solution and make sure that those directories are bundled in wheels/sdist. I think the solution is to use namespace=True (i.e. find_namespace_packages vs find_packages) but this requires more investigation to be sure.