package-directory compatibility tests

Integration tests for package-directory.

To test a new implementation:

1. Add the implementation name to the harness fidl.
2. Have the harness return the new implementation when requested.
3. Have the tests request the new implementation from the harness.
4. If the new implementation has intentionally different behavior, add a method to PackageSource to query the implementation, then have the test branch on the implementation and document the difference in this README.