Use alias for test_per_src apex dependencies

AddFarVariationDependencies was broken, which allowed apex to add
dependencies on tests by requesting an empty test_per_src variation
even though some test dependencies did not have a test_per_src
variation.  Add an alias from the pre-test_per_src variation
variant to the empty test_per_src variation, and drop the
test_per_src variation when requesting the depencency.

Test: TestApexWithTests
Change-Id: I2ed7bdd761027956141e25d4d81f9074afe628b6