Simplify StagedApexInfo 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 ApexTestCases
Test: atest ComposTestCase
Change-Id: I8e66add5ba9a9494d8613a6cf59f78ffa82147f9