Add documentation for the packaging module.

This updates the user guide to refer to Packaging instead of Distutils.
Some files still require an update.
diff --git a/Doc/library/python.rst b/Doc/library/python.rst
index b67fbfc..07eadb4 100644
--- a/Doc/library/python.rst
+++ b/Doc/library/python.rst
@@ -25,4 +25,5 @@
    inspect.rst
    site.rst
    fpectl.rst
+   packaging.rst
    distutils.rst