Instead of writing it by hand.

…umbing`.

We are in the process of eliminating `rustc_query_system`. Chunks of it
are unused by `rustc_middle`, and so can be moved into
`rustc_query_info`. This commit does some of that.

Mostly it's just moving code from one file to another. There are a
couple of non-trivial changes.

- `QueryState` and `ActiveKeyStatus` must remain in `rustc_query_system`
  because they are used by `rustc_middle`. But their inherent methods
  are not used by `rustc_middle`. So these methods are moved and
  converted to free functions.

- The visibility of some things must increase. This includes `DepGraphData`
  and some of its methods, which are now used in `rustc_query_impl`.
  This is a bit annoying but seems hard to avoid.

What little is left behind in
`compiler/rustc_query_system/src/query/plumbing.rs` will be able to
moved into `rustc_query_impl` or `rustc_middle` in the future.

The previous commit moved some code from `rustc_query_system`, which
doesn't have access to `TyCtxt` and `QueryCtxt`, to `rustc_query_impl`,
which does. We can now remove quite a bit of indirection.

- Three methods in `trait QueryContext` are no longer needed
  (`next_job_id`, `current_query_job`, `start_query`). As a result,
  `QueryCtxt`'s trait impls of these methods are changed to inherent
  methods.

- `qcx: Q::Qcx` parameters are simplified to `qcx: QueryCtxt<'tcx>`.

- `*qcx.dep_context()` occurrences are simplified to `qcx.tcx`, and
  things like `qcx.dep_context().profiler()` become `qcx.tcx.prof`.

- `DepGraphData<<Q::Qcx as HasDepContext>::Deps>` becomes
  `DepGraphData<DepsType>`.

In short, various layers of indirection and abstraction are cut away.
The resulting code is simpler, more concrete, and easier to understand.
It's a good demonstration of the benefits of eliminating
`rustc_query_system`, and there will be more to come.