Missing rosidl type support for custom message in Python [Eloquent]

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What I am trying to achieve: within my workspace, create a ament_cmake package with custom messages called navigation_msgs and publish them in an ament_python package.

What works: ros2 interface show navigation_msgs/EulerAttitude correctly displays my message. I am also able to import the message in Python from navigation_msgs.msg import EulerAttitude. Furthermore, within the same package I can publish built-in messages like geometry_msgs (Vector3)

What does not work: when trying to ros2 run pub_node not_working I get a typesupport error (the trace is below). It is raised when I call create_publisher in the NotWorkingNode constructor.

Setup: Ubuntu 18.04 with ROS3 Eloquent from APT. ros2 doctor works. (in fact the problem if the same with Dashing, also from APT.)

How to reproduce: attached code is a minimal workspace with ros2 run pub_node working and ros2 run pub_node working. working should work and notworking should give the error.

TCantelobre@1044617:~/bug$ ros2 run pub_node  notworking 
 Traceback (most recent call last):
     File "/opt/ros/eloquent/lib/python3.6/site-packages/rosidl_generator_py/import_type_support_impl.py", line 40, in import_type_support
        return importlib.import_module(module_name, package=pkg_name)
     File "/usr/lib/python3.6/importlib/__init__.py", line 126, in import_module
        return _bootstrap._gcd_import(name[level:], package, level)
     File "<frozen importlib._bootstrap>", line 994, in _gcd_import
     File "<frozen importlib._bootstrap>", line 971, in _find_and_load
     File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked
     File "<frozen importlib._bootstrap>", line 658, in _load_unlocked
     File "<frozen importlib._bootstrap>", line 571, in module_from_spec
     File "<frozen importlib._bootstrap_external>", line 922, in create_module
     File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  ImportError: /home/TCantelobre/bug/install/navigation_msgs/lib/libnavigation_msgs__rosidl_typesupport_fastrtps_c.so: undefined symbol: rosidl_typesupport_fastrtps_c__get_message_type_support_handle__std_msgs__msg__Header

  During handling of the above exception, another exception occurred:

  Traceback (most recent call last):
     File "/home/TCantelobre/bug/install/pub_node/lib/pub_node/notworking", line 11, in <module>
load_entry_point('pub-node==0.0.0', 'console_scripts', 'notworking')()
     File "/home/TCantelobre/bug/install/pub_node/lib/python3.6/site-packages/pub_node/not_working.py", line 28, in main
not_working_node = NotWorkingNode()
     File "/home/TCantelobre/bug/install/pub_node/lib/python3.6/site-packages/pub_node/not_working.py", line 16, in __init__
self.publisher = self.create_publisher(EulerAttitude, "attitude", 100)
     File "/opt/ros/eloquent/lib/python3.6/site-packages/rclpy/node.py", line 1094, in create_publisher
check_for_type_support(msg_type)
     File "/opt/ros/eloquent/lib/python3.6/site-packages/rclpy/type_support.py", line 29, in check_for_type_support
msg_type.__class__.__import_type_support__()
     File "/home/TCantelobre/bug/install/navigation_msgs/lib/python3.6/site-packages/navigation_msgs/msg/_euler_attitude.py", line 27, in __import_type_support__
          module = import_type_support('navigation_msgs')
     File "/opt/ros/eloquent/lib/python3.6/site-packages/rosidl_generator_py/import_type_support_impl.py", line 42, in import_type_support
          raise UnsupportedTypeSupport(pkg_name)
 rosidl_generator_py.import_type_support_impl.UnsupportedTypeSupport: Could not import 'rosidl_typesupport_c' for package 'navigation_msgs'