Fix apex to follow deps of modules with stubs.

In case that an apex module depends on a module with stubs directly
*and* indirectly, the build system should follow the deps graph further.

Note that WalkDeps() visits deps in DFS and it won't visit again visited
modules.

Bug: n/a
Test: m (apex_test.go amended)
Change-Id: I1a6f135dbda6d1eb641575a3fbbc9bbee0622076