"ImportError no module named" error when package with msg also has setup.py and installed

edit

I have a package with a foo.msg and a python module my_package/src/my_package/bar.py that works fine in the devel space- I can import the msg or alternatively import the python module just fine. But after running catkin_make install the I get an import error:

catkin_make install
source devel/setup.bash
python
>>>> from mypackage.msg import Foo
>>>>
>>>>  # (no error output, worked fine)
source install/setup.bash
>>>> from mypackage.msg import Foo
Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
Import Error: No module named my_package.msg

I commented out the catkin_python_setup from the CMakeLists.txt and rebuilt (rm -rf build devel install followed by source /opt/ros/jade/setup.bash seems safest which maybe ought to become the topic of another question...), then the import works fine.

One thing I notice is that the module bar.py is installed into install/lib/python2.7/dist-packages/my_package, and there are no __init__.py files in that directory, only one directory lower in msg (it contains __init__.py and _Foo.py and pyc versions).

This is jade with Ubuntu 14.04.