[ROS2] Could not import 'rosidl_typesupport_c' for package

edit

I have a set of packages built using ament_python in a workspace and have been trying to create a separate package, built using ament-cmake to contain custom message and service types, as recommended by a couple of different posts I've found.

I'm using ROS2 Foxy on Ubuntu 20.04.

From colcon build --symlink--install, I get no errors and all packages, including the custom interface package I created, build successfully. But when I then ros2 run ot_ctrl tfpub to run a package using a custom interface I get the following error:

rosidl_generator_py.import_type_support_impl.UnsupportedTypeSupport: Could not import 'rosidl_typesupport_c' for package 'qcm_interfaces'

What is happening is that one of the other python-built packages ot_ctrl attempt to reference a custom interface: OTCommand.srv inside my custom cmake-built interface package qcm_interfaces. So I have:

In package.xml:

<exec_depend>qcm_interfaces</exec_depend>

In the node with the service tf_publisher.py:

from qcm_interfaces.srv import OTCommand
...
< within __init__ >
    self._move_srv = self.create_service(OTCommand, 'move', self.move_callback)

Here is my folder structure:

And the full error output:

   Traceback (most recent call last):

  File "/opt/ros/foxy/lib/python3.8/site-packages/rosidl_generator_py/import_type_support_impl.py", line 46, in import_type_support
    return importlib.import_module(module_name, package=pkg_name)
  File "/usr/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 973, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'qcm_interfaces.qcm_interfaces_s__rosidl_typesupport_c'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/ubuntu/dev_ws/install/ot_ctrl/lib/ot_ctrl/tfpub", line 11, in <module>
    load_entry_point('ot-ctrl', 'console_scripts', 'tfpub')()
  File "/home/ubuntu/dev_ws/build/ot_ctrl/ot_ctrl/tf_publisher.py", line 215, in main
    nodes.append(OTController())
  File "/home/ubuntu/dev_ws/build/ot_ctrl/ot_ctrl/tf_publisher.py", line 147, in __init__
    self._move_srv = self.create_service(OTCommand, 'move', self.move_callback)
  File "/opt/ros/foxy/lib/python3.8/site-packages/rclpy/node.py", line 1295, in create_service
    check_for_type_support(srv_type)
  File "/opt/ros/foxy/lib/python3.8/site-packages/rclpy/type_support.py", line 29, in check_for_type_support
    msg_type.__class__.__import_type_support__()
  File "/home/ubuntu/dev_ws/install/qcm_interfaces/lib/python3.7/site-packages/qcm_interfaces/srv/_ot_command.py", line 298, in __import_type_support__
    module = import_type_support('qcm_interfaces')
  File "/opt/ros/foxy/lib/python3.8/site-packages/rosidl_generator_py/import_type_support_impl.py", line 48, in import_type_support
    raise UnsupportedTypeSupport(pkg_name)
rosidl_generator_py.import_type_support_impl.UnsupportedTypeSupport: Could not import 'rosidl_typesupport_c' for package 'qcm_interfaces'