The main changes here are to leverage PackageError and PackageName more.
I also fixed a bug I introduced by misunderstanding code in auto_deps. auto_deps should return PackageNames instead of Strings, but I converted the Strings too early.
This deduplicates code as well as forces names to be sanitized. The API
for both redox-pkg and cookbook need some reimagining due to the
reshuffling, but this patch is not concerned with that as yet.
