lib features ending in '_internals?' are internal

tests/ui/lint/internal_features.rs

@@ -0,0 +1,11 @@
+#![forbid(internal_features)]
+// A lang feature and a lib feature.
+#![feature(intrinsics, panic_internals)]
+//~^ ERROR: internal
+//~| ERROR: internal
+
+extern "rust-intrinsic" {
+    fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
+}
+
+fn main() {}