Fix the declaration and documentation of
`gi_object_info_find_method_using_interfaces()` and
`gi_object_info_find_vfunc_using_interfaces()`. The documentation was
wrong when I wrote it: the value returned is the object or interface
which declares the method or vfunc, not the one which implements it.

The returned declarer info may be a `GIObjectInfo` or a
`GIInterfaceInfo`. Since those two types have no subtype relation
between them, the return type has to be changed to `GIBaseInfo`. Using
`GIObjectInfo` would have been fine in girepository-1.0 because all
`*Info` types were aliases of each other — but since the move to
`GTypeInstance` this is no longer true.

A unit test will be in the following commit.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Fixes: #3246