An alternate to D14839, implemented in Python and
relying on bpy.data.user_map(). That function
gives us a mapping of what ID is referenced by
what set of IDs. The inverse of this would also
be useful, which is now available from
bpy_extras.id_map_utils.get_id_reference_map().
From there, we can use get_all_referenced_ids()
to get a set of all IDs referenced by a given ID
either directly or indirectly.
To get only the direct references, we can simply
pass the ID of interest as a key to the dictionary
returned from get_id_reference_map().
Reviewed By: mont29
Differential Revision: https://developer.blender.org/D14843
