Move Query to rustc_query_system.

Rename it to QueryStackFrame and document a bit.

compiler/rustc_query_impl/src/lib.rs

@@ -17,7 +17,6 @@ extern crate rustc_middle;
 extern crate tracing;
 
 use rustc_data_structures::fingerprint::Fingerprint;
-use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_errors::{Diagnostic, Handler, Level};
 use rustc_hir::def_id::CrateNum;