migrate the C++ _<name> modules directly into the dune namespace

This allows them to be loaded without loading the corresponding Python
mdoule, avoiding circular dependencies.

CMakeLists.txt

 add_python_targets(typeregistry
   __init__
 )
-
-dune_add_pybind11_module(NAME _typeregistry)

__init__.py

-from ._typeregistry import *
+from .._typeregistry import *

_typeregistry.cc

-// -*- tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
-// vi: set et ts=4 sw=2 sts=2:
-#include <config.h>
-
-
-#include <dune/python/common/typeregistry.hh>
-
-
-PYBIND11_MODULE( _typeregistry, module )
-{
-  Dune::Python::registerTypeRegistry(module);
-}