Work-conserving behavior when multiple alternative collections overlap

[intelfx]

A nice feature would be to avoid repeated transcoding if a version of file in proper format already exists in one of the alternative collections.

On update, the plugin could check if any of the to-be-transcoded files are supposed be present in any existing alternative locations (by intersecting the database queries). If the query yields something, try to copy these files from the guessed location. If the file is actually not there, fall back to transcoding.