Missing deps

Implements feature to be used in builder as discussed
@movitto Agreed, naming is hard.  I can see the use case for dependency_versions to provide the complete dependencies meeting the version constraint.

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... maybe the use case can give us good names and good specs that can either refine the abstractions or just clarify the names.

I'd rather not write code for "potential" situations we don't fully understand.  We should let actual use cases drive our development, especially if we have the basic dependency_versions method that provides the "core functionality".  

One of my favorites lines regarding code we "might" need is ["You'll never know less than you know right now"](https://twitter.com/sandimetz/status/487095194538745857).