python setup_helper ignores --install-base flag

[emulate-jonathanmanning]

While setup_helper.generate_parameter_module parses args and respects --build-base, it assumes the default 'install' path and ignores the --install-base argument.

For example:

colcon build --merge-install --install-base=/opt/alt-install-path-v0

Results in the ROS python package being installed to:

/opt/alt-install-path-v0/lib/python3.10/site-packages/foo/...

But the generated code gets left behind all by itself in the workspace

$ tree install/
install/
└── foo
    └── lib
        └── python3.10
            └── site-packages
                └── foo
                    ├── foo_parameters.py

I believe this is different but related to the churn and discussion of similar issues in #207 #236 #237

[christophfroehlich]

I vibe-coded a possible fix for this #348. @emulate-jonathanmanning could you have a look, test it, and leave a review if the GPT-generated code makes sense?