Fix outdated crate names in compiler docs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_lint/src/builtin.rs

@@ -2280,7 +2280,7 @@ declare_lint! {
 }
 
 declare_lint_pass!(
-    /// Check for used feature gates in `INCOMPLETE_FEATURES` in `librustc_feature/active.rs`.
+    /// Check for used feature gates in `INCOMPLETE_FEATURES` in `rustc_feature/src/active.rs`.
     IncompleteFeatures => [INCOMPLETE_FEATURES]
 );