[melodic]: Switch dependency back to python-rosdep.

[clalancette]

Prior to #109 , installing
ros-melodic-desktop on Melodic installed the rosdep command-line.
After that change, it no longer does. For backwards compatibility
reasons, revert that change just for melodic.

Signed-off-by: Chris Lalancette clalancette@openrobotics.org

If we don't want to change this dependency back, I'd suggest that the other place to make this change is in https://github.com/ros/metapackages/blob/melodic-devel/ros_core/package.xml . @dirk-thomas thoughts?

[dirk-thomas]

This package does not use any of the rosdep command line tools and therefore shouldn't depend on it.

The same goes for the metapackage - nothing at runtime requires rosdep to be present so it should not be a dependency.

Instead the instructions should be updated to not rely on transitive dependencies.

[clalancette]

Instead the instructions should be updated to not rely on transitive dependencies.

But the thing is, this is a breaking change for Melodic. For Noetic, sure; we can just say from the beginning that you need to install rosdep now. Breaking backwards compatibility like this for a 2 year old distribution just seems like the wrong thing to do here.

[dirk-thomas]

The only think broken are the instructions. Relying on transitive dependencies is not something which should be done. If dependencies are changed that is exactly what will happen.