Do not throw away backpack instantiations for module lookup cache

Currently, there is only one home package so this probably doesn't
matter. But if we support multiple home packages, they could differ only
in arguments (same indef component being applied).

It looks like it used to be this way before
4e8a0607, but that commit doesn't seem
to comment on this change in the particular. (It's main purpose is
creating the InstalledUnitId and recategorizing the UnitId expressions
accordingly.)

Trying this as a separate commit for testing purposes. I leave it to
others to decide whether this is a good change on its own.