Fix outdated crate names in compiler docs

Changes `librustc_X` to `rustc_X`, only in documentation comments.
Plain code comments are left unchanged.

Also fix incorrect file paths.

compiler/rustc_trait_selection/src/traits/query/mod.rs

@@ -3,7 +3,7 @@
 //! which makes a canonical query by replacing unbound inference
 //! variables and regions, so that results can be reused more broadly.
 //! The providers for the queries defined here can be found in
-//! `librustc_traits`.
+//! `rustc_traits`.
 
 pub mod dropck_outlives;
 pub mod evaluate_obligation;