Allow `submission.yaml` in example submissions

[niemela]

Allow including a submission.yaml with example submissions.

Note that since adding a file means the submission would be at least two files, it would have to be a directory.

The submission.yaml could contains the keys that glob patterns in submissions.yaml can map to, and these would apply to this submission only, as additional requirements. So, submission.yaml would never override submissions.yaml, only add additional requirements.