The YAML file names have an optional `:label` portion that becomes the
latter part of the filename. Reconstructed paths (eg. from
`serdes-dependencies`) don't have those labels.

This change makes the YAML ingestion code able to find a file with a
human-readable label even if the request didn't include it.
No ambiguity results because the file names are always based on the
unique serdes `:id`, usually an `entity_id`.