Missing deps

Implements feature to be used in builder as discussed
@jrafanie with a single call to the dependency_versions method you are able to query multiple targets as well as multiple 'collections' per target, eg multiple koji tags, multiple git branches, multiple yum repos, etc.

dependency_versions / versions_for / version_of (again names could be changed) provide an abstraction chain which a client can easily tap into at any point. If they want all the dependency versions in all targets they can use 'dependency_versions', if they want the most common version per target they could use 'versions_for' and if they want one overall snapshot of the version of the package currently working its way through multiple channels in the system, they could use 'version_of'. If they want to spin something of their own they can do that as well, potentially leveraging any of these abstraction points.

The data in the specs is just dummy test data / not too make terrible much sense outside the scope of what is being tested (the versions per target and overall are being deduced correctly)