Instead of tracking loaded dependencies ourself to hide warnings if they
were loaded by a previous import just look if the namespace was loaded
before the import. This (a) makes the implementation much simpler and
(b) also takes into account namespaces loaded outside of Python/PyGObject
using the libgirepository C API (as is common in applications using
libpeas with Python plugins)

This also introduces a new Python wrapper for g_irepository_is_registered()
to allow checking the loading state of namespaces before imports.

This fixes unnecessary require_version warnings in gedit, gnome-builder,
totem, rhythmbox etc.

https://bugzilla.gnome.org/show_bug.cgi?id=754491