It was previously cached for modules loaded from `fn get_module`, but not for modules loaded from `fn build_reduced_graph_for_external_crate_res`.
This also makes all foreign modules use their real parent, span and expansion instead of possibly a parent/span/expansion of their reexport.
An ICE happening on attempt to decode expansions for foreign enums and traits is avoided.
Also local enums and traits are now added to the module map.