Second phase of refactoring for runpy, pkgutil, pydoc, and setuptools

Second phase of refactoring for runpy, pkgutil, pydoc, and setuptools
to share common PEP 302 support code, as described here:

http://mail.python.org/pipermail/python-dev/2006-April/063724.html

pydoc now supports PEP 302 importers, by way of utility functions in
pkgutil, such as 'walk_packages()'.  It will properly document
modules that are in zip files, and is backward compatible to Python
2.3 (setuptools installs for Python <2.5 will bundle it so pydoc
doesn't break when used with eggs.)

What has not changed is that pydoc command line options do not support
zip paths or other importer paths, and the webserver index does not
support sys.meta_path.  Those are probably okay as limitations.

Tasks remaining: write docs and Misc/NEWS for pkgutil/pydoc changes,
and update setuptools to use pkgutil wherever possible, then add it
to the stdlib.