Allow multiple values for package_data in setup.cfg (#11805).

Allow multiple values for package_data in setup.cfg (#11805).

Even though the resources system obsoletes data_files and package_data
(see bug discussion), package_data still exists to allow compatibility
with distutils and thus an easier transition.  In setup.py, the values
are lists of glob patterns, so the setup.cfg syntax needed a way to
express multiple values too.

Doc for this option will be added later as part of the big packaging doc
patches.  For now, the test serves as example.

Reported by Erik Bray.