Move parts of rustc_query_system::query::job to rustc_middle::job.

The latter is a new module.

As well as the code motion, some other changes were required.
- `QueryJobId` methods became free functions so they could move while
  `QueryJobId` itself stayed put. This was so `QueryMap` and
  `QueryJobInfo` could be moved.
- Some visibilities in `rustc_query_system` required changing.
- `collect_active_jobs_from_all_queries` is no longer required in `trait
  QueryContext`.

Cargo.lock

@@ -4503,6 +4503,7 @@ dependencies = [
  "rustc_query_system",
  "rustc_serialize",
  "rustc_span",
+ "rustc_thread_pool",
  "tracing",
 ]