Install ign_ros2_control_demos/urdf directory to fix demos

[AndrejOrsula]

All launch scripts under ign_ros2_control_demos require ign_ros2_control_demos/urdf directory to be installed, see example (cart_example_effort.launch.py).

The installation of ign_ros2_control_demos/urdf directory was disabled as a part of #56, which I believe was an unintentional consequence of removing urdf package dependency. Therefore, this PR reverts this change.

Note: This PR targets main but all branches are affected (foxy/galactic).

🦟 Bug fix

Fixes the following error that is experienced when running launch scripts included in ign_ros2_control_demos.

[ERROR] [launch]: Caught exception in launch (see debug for traceback): Caught exception when trying to load file of format [py]: [Errno 2] No such file or directory: '/root/ws_moveit/install/ign_ros2_control_demos/share/ign_ros2_control_demos/urdf/test_cart_position.xacro.urdf'

Summary

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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