Moving to use evgen keywords helper functions in transforms

The helpers seem like a nice way to do the same thing centrally instead of three times, and they avoid the warning currently in some evgen:

generate 03:17:20 2024-10-23 03:17:01,213 AthMpEvtLoopMgr...WARNING The file /cvmfs/atlas-nightlies.cern.ch/repo/sw/main_AthGeneration_x86_64-el9-gcc13-opt/2024-10-22T2126/AthGeneration/23.6.40/InstallArea/x86_64-el9-gcc13-opt/share/evgenkeywords.txt has not been registered with the FileMgr!

Apologies in case there's something I'm missing that makes this a bad idea; I hope @ewelina will say if I've misunderstood the aim of the helpers (note for the RC: please wait for a comment before merging, but I think L1 review can go ahead).