btcpp_ros2_interfaces use msgs instead of msg

[AdrianZw]

If we call ros2 interface show btcpp_ros2_interfaces/msg/NodeStatus, we get the following issue:

Error processing '// generated from rosidl_adapter/resource/msg.idl.em' of 'btcpp_ros2_interfaces/NodeStatus': '//'
Traceback (most recent call last):
  File "/opt/ros/humble/bin/ros2", line 33, in <module>
    sys.exit(load_entry_point('ros2cli==0.18.10', 'console_scripts', 'ros2')())
  File "/opt/ros/humble/lib/python3.10/site-packages/ros2cli/cli.py", line 91, in main
    rc = extension.main(parser=parser, args=args)
  File "/opt/ros/humble/lib/python3.10/site-packages/ros2interface/command/interface.py", line 35, in main
    return extension.main(args=args)
  File "/opt/ros/humble/lib/python3.10/site-packages/ros2interface/verb/show.py", line 201, in main
    _show_interface(
  File "/opt/ros/humble/lib/python3.10/site-packages/ros2interface/verb/show.py", line 148, in _show_interface
    for line in _get_interface_lines(interface_identifier):
  File "/opt/ros/humble/lib/python3.10/site-packages/ros2interface/verb/show.py", line 116, in _get_interface_lines
    yield InterfaceTextLine(
  File "/opt/ros/humble/lib/python3.10/site-packages/ros2interface/verb/show.py", line 43, in __init__
    msg_spec = parse_message_string(
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/rosidl_adapter/parser.py", line 520, in parse_message_string
    Type(type_string, context_package_name=pkg_name),
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/rosidl_adapter/parser.py", line 277, in __init__
    super(Type, self).__init__(
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/rosidl_adapter/parser.py", line 190, in __init__
    raise InvalidResourceName(type_string)
rosidl_adapter.parser.InvalidResourceName: //

The root cause is that the NodeStatus.msg is located and installed into msgs, but msg is expected, see
https://docs.ros.org/en/humble/Tutorials/Beginner-Client-Libraries/Custom-ROS2-Interfaces.html#cmakelists-txt

[ldegen]

FWIW, I can confirm that the changes introduced in #81 fixed the problem for me. Thx @AdrianZw