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/create.py

@@ -36,7 +36,7 @@ from packaging._trove import all_classifiers as _CLASSIFIERS_LIST
 from packaging.version import is_valid_version
 
 _FILENAME = 'setup.cfg'
-_DEFAULT_CFG = '.pypkgcreate'
+_DEFAULT_CFG = '.pypkgcreate'  # FIXME use a section in user .pydistutils.cfg
 
 _helptext = {
     'name': '''
@@ -127,6 +127,10 @@ def ask_yn(question, default=None, helptext=None):
         print('\nERROR: You must select "Y" or "N".\n')
 
 
+# XXX use util.ask
+# FIXME: if prompt ends with '?', don't add ':'
+
+
 def ask(question, default=None, helptext=None, required=True,
         lengthy=False, multiline=False):
     prompt = '%s: ' % (question,)