Prefetch fixed cookbooks

**NOTE** Dependency on cookbook-omnifetch will be updated to once that's released.

Adds limited ability to cache cookbooks so that git and local cookbook data can be fed into the dependency solver. With this patch it is possible to evaluate a policyfile and produce a solved dependency graph.
Ideally I can just switch to supermarket and not worry about anything else. But also, for now at least, Policyfile supports one default location and any cookbooks from different sources have to be explicitly specified, so most of this data is irrelevant and I just throw it out.