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

First 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

This revision strips all the PEP 302 emulation code from runpy,
replacing it with published API classes and functions in pkgutil,
mostly using setuptools' implementation of common functionality,
but adding features from runpy, and doing some refactoring to make
the layer pydoc needs easier to implement on top of this.

One step down, four to go, although step #4 (adding C versions of
the new APIs to 'imp') may not be able to make it in time for
alpha 2.  We'll see how that goes.