It may happen that a Gio method (such as Gio.Subprocess.get_stdout_pipe())
returns an object from GioUnix. We need to handle that case in four
places: gjs_lookup_object_prototype(), gjs_lookup_object_constructor(),
gjs_lookup_interface_constructor(), and gjs_value_from_g_value_internal().

Introduce a new method gjs_lookup_gtype() that calls
g_irepository_find_by_gtype() and then performs the necessary adjustments
afterwards.

Closes: #610