Missing deps

Implements feature to be used in builder as discussed
Ya, introducing a bit of dependency abstraction in polisher would make sense. Since it adds a bit of complexity, perhaps we can do this in a separate PR after this is merged? 

The local requirement is easy enough to be deduced if just the packages are returned, eg
polisher_gemfile.dependency_for(pkg). But the 'found' versions would need to be queried for again unless returned here, a potentially costly operation (depending on config, network bandwidth, etc)