DRY-up the python package handling code

[ifireball]

There is some repetition in the code handling the Python modules, it could probably be re-factored into a PythonPackage class.
That class could probably include an enhanced version of 'name_from_class()' that would remove the 'Python' prefix so that, for example, the PythonQREncode would transparently refer to the 'qrencode' pip package.