fix: merge install for python packages

[darrenjkt]

Existing install of the parameter python script fails when using colcon build with --merge-install and/or --install-base /opt/someuser/ros_ws

This PR enables us to pass through a custom install base e.g. /opt/someuser/ros_ws as well as the merge-install flag for proper installation of python packages in the same way as colcon build.

One way to obtain the merge install path can be done as below and passed into generate_parameter_module.

if len(sys.argv) >= 2 and sys.argv[1] != 'clean':
    from generate_parameter_library_py.setup_helper import generate_parameter_module
    
    colcon_prefix_path = os.getenv("COLCON_PREFIX_PATH", None)
    if not colcon_prefix_path:
        raise ValueError(f"Unable to find merge install base")    
    base_dir = colcon_prefix_path.split(':')[0] 

    generate_parameter_module(
        module_name="detector_onnx_parameters", 
        yaml_file="detector_onnx/detector_onnx_parameters.yaml", 
        install_base=base_dir,
        merge_install=True
    )

With this PR, running the following below commands in /home/rosuser/ros_ws (and editing the args above respectively), will place the auto-gen parameter script in the following:

> colcon build
/home/rosuser/ros_ws/install/my_package/lib/python3.12/site-packages/my_package/my_package_parameters.py

> colcon build --install-base /opt/myuser/ros_ws
/opt/myuser/ros_ws/my_package/lib/python3.12/site-packages/my_package/my_package_parameters.py

> colcon build --merge-install 
/home/rosuser/ros_ws/install/lib/python3.12/site-packages/my_package/my_package_parameters.py

> colcon build --merge-install --install-base /opt/myuser/ros_ws 
/opt/myuser/ros_ws/lib/python3.12/site-packages/my_package/my_package_parameters.py