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.

The right-hand part in [extension: foo] is now used as the name of the
extension module.  (I changed the separator from = to : and allowed
whitespace to make the sections look nicer.)

This huge module is the heir of six distutils modules, and contains
a number of miscellaneous functions.  I have attempted to help readers
of the source code with an annoted __all__.  Removed or deprecated
functions have been removed from the documentation; I’m working on
another patch to document the remaining public functions.

For the curious:

The unzip_file and untar_file were used by (or intended to be used by)
“pysetup install path/to/archive.tar.gz”, but the code presently used
shutil.unpack_archive and an helper function, so I just deleted them.
They’re still in the repository if we need them in the future.

The find_packages function is not used anymore but I want to discuss
module and package auto-discovery (in “pysetup create”) again before
removing it.

subst_vars now lives in sysconfig; rfc822_escape is inlined in
packaging.metadata.  Other functions are for internal use only, or
deprecated; I have left them out of __all__ and sprinkled TODO notes
for future cleanups.

Original patch by Vinay Sajip on #11637.

Original patch by Erik Bray.

Original patch by Erik Bray as part of #11595, changed by me to improve
readability.