Clean up packaging.util: add __all__, remove some unused functions.

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.

Lib/packaging/command/build_py.py

@@ -393,7 +393,7 @@ class build_py(Command, Mixin2to3):
                            self.get_command_name())
             return
 
-        from packaging.util import byte_compile
+        from packaging.util import byte_compile  # FIXME use compileall
         prefix = self.build_lib
         if prefix[-1] != os.sep:
             prefix = prefix + os.sep