syntax: Bless mod.rs. #4116

When loading a module the parser will look for either foo.rs or foo/mod.rs and generate
an error when both are found.

src/test/run-pass/mod_dir_implicit.rs

@@ -0,0 +1,18 @@
+// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// xfail-pretty
+// xfail-fast
+
+mod mod_dir_implicit_aux;
+
+pub fn main() {
+    assert_eq!(mod_dir_implicit_aux::foo(), 10);
+}