rustdoc: Stop stripping empty modules

There is no good reason to strip empty modules with no documentation and
doing so causes subtle problems.

src/test/rustdoc/empty-mod-public.rs

@@ -0,0 +1,24 @@
+// Copyright 2017 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.
+
+// @has 'empty_mod_public/index.html' '//a[@href="foo/index.html"]' 'foo'
+// @has 'empty_mod_public/sidebar-items.js' 'foo'
+// @matches 'empty_mod_public/foo/index.html' '//h1' 'Module empty_mod_public::foo'
+pub mod foo {}
+
+// @has 'empty_mod_public/index.html' '//a[@href="bar/index.html"]' 'bar'
+// @has 'empty_mod_public/sidebar-items.js' 'bar'
+// @matches 'empty_mod_public/bar/index.html' '//h1' 'Module empty_mod_public::bar'
+pub mod bar {
+    // @has 'empty_mod_public/bar/index.html' '//a[@href="baz/index.html"]' 'baz'
+    // @has 'empty_mod_public/bar/sidebar-items.js' 'baz'
+    // @matches 'empty_mod_public/bar/baz/index.html' '//h1' 'Module empty_mod_public::bar::baz'
+    pub mod baz {}
+}