Allow overrides in other directories than gi itself

Use pkgutil.extend_path() for the gi and gi.overrides modules, so that
libraries can install overrides in a path that is different from the one that
pygobject installs itself into. These overrides need to put this into their
__init__.py at the top:

    from pkgutil import extend_path
    __path__ = extend_path(__path__, __name__)

and put themselves somewhere into the default PYTHONPATH.

https://bugzilla.gnome.org/show_bug.cgi?id=680913



Co-Authored-By: Martin Pitt <martinpitt@gnome.org>
Co-Authored-By: Simon Feltman <s.feltman@gmail.com>