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.

[mmitche]

This might be harder than originally thought, unless Maestro is just adding every ID from the feed, since repos will use one property to represent the version of multiple packages.

[premun]

Can't we get all the assets from the currently flown build and find the longest common prefix or something to put in the PSM?