Auto merge of #8068 - ehuss:bcx-units, r=alexcrichton,ehuss

Auto merge of #8068 - ehuss:bcx-units, r=alexcrichton,ehuss

Refactor BuildContext

This restructures the "front end" of the compile process so that the `UnitGraph` can be accessed by API users. Essentially, the `BuildContext` contains the result of generating the `UnitGraph`, and other bits of information collected along the way. This logically separates the build process into two phases: (1) generate the `UnitGraph` and `BuildContext` and (2) pass the `BuildContext` to `Context` which performs the actual compilation.

The main challenge here is dealing with the references and lifetimes. The old code kept a bunch of things on the stack with various layers of references. Beside reorganizing things, the big change is to wrap `Package` and `Target` in `Rc`. This still requires the `UnitInterner` to be passed in and kept alive. It is possible to avoid that by placing all `Unit`s in `Rc`, but that had a roughly 5% performance hit (on fresh builds) because Units are very optimized to be used as hashable keys, and `Rc` loses those optimizations.