Support references defined in included files

References are currently supported when they are defined in the same yaml file.

It would be great to have support for references stored in included files.
Example can be found here in gitlab docs

Current implementation raises exception:

File "/tmp/test-gle-refs/.venv/lib/python3.9/site-packages/gitlabemu/yamlloader.py", line 63, in reference_constructor
    raise ReferenceError(f"cannot find referent job for {reference} at {node.start_mark}")

gitlabemu.yamlloader.ReferenceError: cannot find referent job for !reference [.setup, script] at   in "/tmp/test-gle-refs/.gitlab-ci.yml", line 10, column 7