Revert "Modules are either public, or private, so private modules should be stripped out"

This reverts commit 7180b5de44.

src/librustdoc/passes.rs

@@ -128,8 +128,7 @@ fn fold_item(&mut self, i: Item) -> Option<Item> {
                 }
             }
 
-            clean::ViewItemItem(..) |
-            clean::ModuleItem(..) => {
+            clean::ViewItemItem(..) => {
                 if i.visibility != Some(ast::Public) {
                     return None
                 }
@@ -141,6 +140,9 @@ fn fold_item(&mut self, i: Item) -> Option<Item> {
                 }
             }
 
+            // handled below
+            clean::ModuleItem(..) => {}
+
             // trait impls for private items should be stripped
             clean::ImplItem(clean::Impl{ for_: clean::ResolvedPath{ id: ref for_id, .. }, .. }) => {
                 if !self.exported_items.contains(for_id) {