This is a sibling of atlasexternals!595 (merged). It only works with the code proposed in that MR. (Hence it's a WIP, as it wouldn't work as-is. I have additional changes locally, which make sure that my test build uses https://gitlab.cern.ch/akraszna/atlasexternals/tree/CMakeUpdates-master-20200106.
The most important change is that
atlas_project(...) no longer receives the name and version of a project. Those now have to be specified in a call to project(...) at the top of the project
CMakeLists.txt files. However the version numbers, in practice, are still read in from the same
version.txt files as before. So for people not regularly editing the project
CMakeLists.txt files, this should not cause any difference.
Most of the documentation / discussion about these updates is in atlasexternals!595 (merged).