Handle `test_per_src` test variations as dependencies of APEX modules.

If a test module with a `test_per_src` property set to `true` is
included in an APEX module, add all the variants for mutator
`test_per_src` as dependencies of the APEX module (not just the
first one).

This is done by adding variation "" of mutator `test_per_src` when
adding a test dependency to an APEX module, which creates an indirect
dependency of the APEX module on all the `test_per_src` variants of
the test module. When generating outputs for the APEX bundle, fetch
and include the set of test outputs from the "" variant.

Test: m (`apex/apex_test.go` amended)
Bug: 129534335
Change-Id: I1c99855971a8a9b2fc5b964a420e882b6791d4e6