Simplify ApexInfo classpath info

Stop differentiating between the different ways an APEX can contribute
to derive_classpath - it makes no difference for the use cases, and it
doesn't scale if new environment variables are added.

Bug: 210472252
Test: atest ComposTestCase
Change-Id: I9256a6b5cc3689a8d61d51e8eb8d8c3f475172aa