Fix outdated crate names in compiler docs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_middle/src/ich/impls_syntax.rs

@@ -1,5 +1,5 @@
 //! This module contains `HashStable` implementations for various data types
-//! from librustc_ast in no particular order.
+//! from `rustc_ast` in no particular order.
 
 use crate::ich::StableHashingContext;