Split `ResolutionGraph::from_state` into methods (#5439)
Two changes split out from the instability work:
* Break `ResolutionGraph::from_state` into methods before adding new
logic to it.
* `ResolutionGraph`: Convert `NodeKey` type to `PackageRef` struct:
Another small refactoring to make subsequent changes easier.
No functional changes.
@BurntSushi I hope this doesn't interfere with your work too much, the
`PackageRef` should at least make debugging panics here easier.
