package-directory compatibility tests

Integration tests for package-directory.

To run this test suite against a package directory backed by a different implementation, you have to create a new puppet component to serve that implementation. Follow these steps:

1. Create a new ./{name}-puppet directory (example: ./pkgdir-puppet).
2. Implement a new puppet component (see the source code from the previous example).
3. Create a new fuchsia_package("{name}-integration-tests") { .. } target with the new puppet as a subpackage. meta/test-root.cml will look for the puppet component at the subpackage URL puppet#meta/default.cm