Auto merge of #1991 - carols10cents:pipelinify, r=alexcrichton

Auto merge of #1991 - carols10cents:pipelinify, r=alexcrichton

Hi! This is an attempt to start refactoring some of the internals to be more like a pipeline, and eventually enable the kind of functionality I tried to add in #1968 without having to add as much duplication. Turns out there's a fair bit of duplication in the code today, I think this helps address it!

I may have totally gone against some abstractions... namely I made a way to create `Package`s from a `manifest_path` and a `config`, without needing a `Source`. I think it cleans up the code quite a bit, and I think makes things a bit more pipeliney in that the `Source` isn't updated until we really need it to be (as opposed to having to use `preload` to avoid updating it again). But I'm open to the possibility that I'm moving things around to where no one who knows the code well will be able to find them ;)

This *should* be a Real Refactor in the sense that these changes don't change behavior-- except in one test case, where the same error happens as did before, but it's going through a `chain_error` now so has a slightly different message.