Auto merge of #2022 - alexcrichton:refactor, r=brson

This series of commits performs two primary tasks:

* The backend is refactored (again!) to have a much better understanding of what's used as input to everything else in terms of parallelization and dependency tracking. The major goal here was to completely unify all parts of the backend which sort of ad-hoc track dependencies and deal with cross compilation. More details can be found in the first commit message.
* Greatly improving diagnostics for why a crate is being rebuilt. This still requires setting `RUST_LOG`, but it was spurred on by #2011 and should help at least my own personal internal debugging of "why is this crate rebuilding?" which seems to come up quite often!