Using head.hackage

Dear GHC devs, As part of our work in "quick look impredicativity", we would like to test whether any package breaks. I've read about "head.hackage" as a way to test this fact, but I could not find any information. Could somebody point me in the right direction? If it does not exist yet, I'll try to summarize whatever I learn in the wiki for future reference. Thanks in advance, Alejandro