[fuzzing] Targets/packages don't have to be colocated

This CL allows fuzz_targets to be in different GN files than the
fuzz_packages that include them, e.g. a fuzz_package could have:
  targets = [ "subdir:some_target" ]

SEC-102

Tests: Built overnet fuzzer
Change-Id: I8bd6df153937d0088de9d1a60b5c9855e6e4fed4
1 file changed