python import path for sub modules if put in namespace package

I have a python modules written in C, it has a main module and a submodule(name with a dot, not sure this can be called real submodule):

PyMODINIT_FUNC initsysipc(void) {
    PyObject *module = Py_InitModule3("sysipc", ...);
    ...
    init_sysipc_light();
}

static PyTypeObject FooType = { ... };
PyMODINIT_FUNC init_sysipc_light(void) {
    PyObject *module = Py_InitModule3("sysipc.light", ...);
    ...
    PyType_Ready(&FooType);
    PyModule_AddObject(module, "FooType", &FooType);
}

The module is compiled as sysipc.so, and when I put it in current directory, following import works without problem:

import sysipc
import sysipc.light
from sysipc.light import FooType

The problem is I want to put this module inside a namespace package, the folder structure is like this:

company/
company/__init__.py
company/dept/
company/dept/__init__.py
company/dept/sys/
company/dept/sys/__init__.py
company/dept/sys/sysipc.so

all the three __init__.py just includes the standard setuptool import line:

__path__ = __import__('pkgutil').extend_path(__path__, __name__)

in current directory, following imports does not work:

from company.dept.sys import sysipc;
from company.dept.sys.sysipc.light import FooType;

How should I import the types and methods defined in module sysipc.light in this case?

from python import path for sub modules if put in namespace package