Fix outdated crate names in compiler docs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_ast_lowering/src/lib.rs

@@ -93,7 +93,7 @@ struct LoweringContext<'a, 'hir: 'a> {
 
     /// HACK(Centril): there is a cyclic dependency between the parser and lowering
     /// if we don't have this function pointer. To avoid that dependency so that
-    /// librustc_middle is independent of the parser, we use dynamic dispatch here.
+    /// `rustc_middle` is independent of the parser, we use dynamic dispatch here.
     nt_to_tokenstream: NtToTokenstream,
 
     /// Used to allocate HIR nodes.