Automatically add files referenced by configuration to sdist

[abravalheri]

This PR is an alternative to #3771. Both solve the same problem, but use different implementations...

Summary of changes

• Add a "protected" attribute to the Distribution class: _referenced_files
• During config expansion, populate this attribute when the file directive is used
• Use the same approach for pyproject.toml's project.readme and project.license.file.

Closes #2821 #3570

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d/.
  (See documentation for details)

[recordedfuture-dev]

It seems like this change triggers such error:
(on multiple machines, but with the same project)

Using cached six-1.16.0-py2.py3-none-any.whl (11 kB)
Building wheels for collected packages: <package>
  Building wheel for <package> (pyproject.toml) ... error
  error: subprocess-exited-with-error

  × Building wheel for <package> (pyproject.toml) did not run successfully.
  │ exit code: 1
  ╰─> [20 lines of output]
      running bdist_wheel
      running build
      running build_py
      running egg_info
      writing src/<package>.egg-info/PKG-INFO
      writing dependency_links to src/<package>.egg-info/dependency_links.txt
      writing entry points to src/<package>.egg-info/entry_points.txt
      writing requirements to src/<package>.egg-info/requires.txt
      writing top-level names to src/<package>.egg-info/top_level.txt
      reading manifest file 'src/<package>.egg-info/SOURCES.txt'
      writing manifest file 'src/<package>.egg-info/SOURCES.txt'
      /private/var/folders/c4/6462yxf34rv22w3577ab5jt00000gt/T/pip-build-env-rn3twi53/overlay/lib/python3.8/site-packages/setuptools/config/pyprojecttoml.py:108: _BetaConfiguration: Support for `[tool.setuptools]` in `pyproject.toml` is still *beta*.
        warnings.warn(msg, _BetaConfiguration)
      error: Error: setup script specifies an absolute path:

          /

      setup() arguments must *always* be /-separated paths relative to the
      setup.py directory, *never* absolute paths.

      [end of output]

  note: This error originates from a subprocess, and is likely not a problem with pip.
  ERROR: Failed building wheel for <package>
Failed to build <package>
ERROR: Could not build wheels for <package>, which is required to install pyproject.toml-based projects

[abravalheri]

Hi @recordedfuture-dev, could you please share which package you are trying to install? (I want to have a look on its pyproject.toml or setup.cfg).