neo_doxygen: Support namespaces’ members

In some languages, like Python, the immediate parent of a member is not necessary a class. So, for cases when namespace as direct members, add an implicit class named `(self)` to list them in a Nit model.

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>
Warning: The last commit will move to the queue of a future PR in order to avoid latter to depend on this one.