Deny internal lints on two more crates

- libfmt_macros
- librustdoc

src/libfmt_macros/lib.rs

@@ -9,6 +9,7 @@
        test(attr(deny(warnings))))]
 
 #![deny(rust_2018_idioms)]
+#![cfg_attr(not(stage0), deny(internal))]
 
 #![feature(nll)]
 #![feature(rustc_private)]