Missing deps

Implements feature to be used in builder as discussed
@jrafanie the names aren't the greatest, but the intent is to deduce the 'most relevant' version of a package determined by the number of references in the configured query targets.

Since any given target can have multiple versions, multiple koji tags, distgit branches, yum repos, etc we need two versions of this, one returning the most relevant version of the package on a per-target basis and the other returning the most relevant version across all targets.