Missing deps

Implements feature to be used in builder as discussed
Agreed, we can do this in a followup pull request as I'm still unsure what would "look" the best from a client perspective, but I do think knowing what gem constraints failed to be matched and what is available would be useful from a client perspective.  We can certainly have a private method that returns a raw Hash/Array with all of the information above and have public methods that call that method and return 1) just the missing dependencies (what is currently implemented here) and a second method 2) that formats the Hash/array into objects I'm proposing above with a cleaner interface.

Let's do that after merging this.