Fix devXlhcb slots
Simulation projects were not working in the devXslots because we were not adding the generators directories to CMAKE_PREFIX_PATH
. This MR fixes the problem.
I also noticed that we used to mask hepmc3 to avoid conflicts, but now dev3 and dev4 should not have hepmc2 anymore and we plan to switch to hepmc3 (see lhcb/LHCb!4885 (merged)), so I fixed that problem too.