Update package_name() to work with namespace pkgs

The logic in pyramid.path.package_name() should take into
account the fact that namespace packages created by
setuptools do not have __init__.py[c] files, and so they
have no __file__ attribute.

This resolves an issue with WSME
(https://bugs.launchpad.net/wsme/+bug/1221201)

Change-Id: I39bc32a9c38fa11c4cef22a041077ed9001091be