Auto merge of #5258 - djc:split-resolver, r=Eh2406

Auto merge of #5258 - djc:split-resolver, r=Eh2406

Split resolver module to make it more manageable

The `core::resolver` module is currently the largest in Cargo, at some 2000 lines. Here's an attempt at splitting it into more reasonable parts and reordering the code in it for better comprehensibility.

Splitting is done in three major steps:

* Move the `Resolve` type and its dependencies into a separate module (~220 lines)
* Move utility data types into a separate module (~400 lines)
* Move the `Context` type and its dependencies into a separate module (~400 lines)

This halves the size of the root module, which is then reordered to make it more readable.

(This is a yak-shaving expedition of sorts, in preparation for #1286.)