Don't magically use implementations for modules in the same apex

To reduce the complexity of the apex logic, remove the apex top down
mutator, and make the build system more understandable, remove the
logic that automatically uses implementations of modules that have
stubs but are in the same apex.  Modules in an apex that want to
depend on the implementations of another module should depend on it
using <dep>#impl.  Misuse of the #impl syntax to depend on a module
that is not in the same will be detected and reported as an error
by the apex when it sees a transitive dependency linked against an
implementation library that is not in the apex.

Bug: 372543712
Test: all apex tests pass
Test: builds
Change-Id: I28a1bb0ca71d5cfeaae145e951c436315aadf5ca