Split 0ad package into pyrogenesis and 0ad

This splits the 0ad package into a pyrogenesis package and a 0ad package. The 0 A.D. executable pyrogenesis can be useful without the 0ad-data package, for example as a way to create new mods for 0 A.D. when used with -archivebuild. Pulling in the 3GB of data for this task is entirely un-necessary, and makes this awkward to use with e.g. a Docker container should one want to package mods on CI (which is something I've wanted to do recently).

As an alternative, the 0ad-data package could be a 'recommended' package, but I think this might lead to more misunderstandings, particularly since the mod mod is in 0ad-data and not 0ad-data-common.

I must apologise for not being very familiar with the debian packaging system, what I've read leads me to believe that this is all that is necessary to change given that the new pyrogenesis package is essentially just 0ad without 0ad-data. Let me know if there are other changes that would make this a more useful contribution.

(FWIW, I am a 0 A.D. developer)