IFD for package.xml

A good quality of life feature would be a best-guess generation of a Nix package description from a ROS2 package.xml. In fact this is how the nix-ros-overlay is generated every so often, using this project: https://github.com/ros-infrastructure/superflore

We should try to use this project to generate Nix definitions on the fly as needed (using Import-From-Derivation or IFD). In the case that a definition cannot be generated, we should provide our best guess to the user and allow them to manually write the definition.

Edit: it seems that somebody had the same idea: https://github.com/wentasah/ros2nix/