Handle stubs within an apex with apex_available

Note this doesn't entirely match Soong's logic but is an improvement to
allow linking against implementation when two cc modules are
apex_available to the same module.

It is not possible to recreate the logic for "directly in" without
significant changes to bp2build as we do not add dependencies nor run
apex mutators. Rather than trying to replicate this, we would be better
off refactoring Soong to no longer support the "directly in apex" logic
and require users to correctly specify apex_available.

Bug: 272378496
Test: go test conversion tests
Change-Id: I17ac426f9b4bdad0c2ab661484e5d994f63568ce