Install scripts properly in symlink installs

[gbiggs]

When using colcon build --symlink-install, the installation method used for Python packages is different. It uses python setup.py develop instead of python setup.py install (see https://setuptools.readthedocs.io/en/latest/setuptools.html#development-mode). This requires a separate section in the setup.cfg file. Without it, the scripts get copied to the <install-prefix>/bin directory instead of <install-prefix>/lib/<pkgname>. They can be executed directly but will not work with ros2 run.

The fix is to add a develop section to setup.cfg that specifies where scripts should be linked to when installing in development mode. See here for an example.