The `namespace_list` helper is used to filter YARD objects in order to populate
search lists. The code was originally derived from the `class_list` helper.
This patch guards against calling `children` on objects that have no children
and removes a recursive call to the old `class_list` method.
The default YARD template lumps anything that inherits from `NamespaceObject`
into the "Classes" list in HTML output. This patch alters the list generation
logic so that Namespaces parsed from manifests are listed in a separate
"Puppet Manifests" list.
Charlie Sharpsteen