ament_cmake_python: Update ament_python_install_package() to allow installing more files into an existing package. #514

It seems like ros2/rosidl_python#141, errors with generating interfaces and using ament_python_install_package(), is blocking the release of some packages. The one I happen to encounter is audio_common: ros-drivers/audio_common#227

Following the discussions, I found this suggestion as a better alternative to a proposed fix in rosidl_python: ros2/rosidl_python#187 (review)

"I think the best option would be to update ament_python_install_package() to allow installing more files into an existing package.

For example,"

rosidl_generate_interfaces(...)

ament_python_install_package(... EXTEND_EXISTING)

Can someone implement this?