Missing deps

Implements feature to be used in builder as discussed
> Maybe if you can come up with some use cases for the versions_for/version_of... why might you call this in day to day packaging/monitoring

One use case is in apps (web, cli, or other) that facilitate a build process and/or upstream/downstream cross-referencing. The versions_for / version_of calls would be used to display a single version representing a high level snapshot of a given package which the user can use to dive into more details.