Can wet package depend on dry package?

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Is the main reason for this is to get rid of rosmake as quick as possible?

The main reason would be so that the new system doesn't have all the terrible hacks the old system had underneath it. The new system is more linux-like with a more standard build/install structure -- making it aware of the older, non-standard approach would cause a massive mess.

So, what if the packages you need are only provided as dry packages? Do you have to convert them first?

Either that, or use rosmake for your package.

The other reason catkin pkgs do not depend on rosmake is that they don't provide all of the necessary exported information for catkin packages to build on top of them automatically. However, rosmake packages do export pkg-config files, which you could try to use, but at your own risk.