darc-* auto-added feeds should have auto-populated package source mappings

[mmitche]

If a repo uses package source mappings, and Maestro is adding a darc-* feed, it should update the darc-* entry with the package source mappings.

Maestro knows what package IDs the darc* feed will contain, since it found which feed to add based on the packages IDs + versions listed in the Version.Details.xml.