Add Moore ConfigurableUser for compatibility with ProdConf
This is an alternative to lhcb-datapkg/ProdConf!19 (closed) . The idea is to try to conform to the old Moore interface expected by ProdConf (at least until ProdConf gets the planned revamp).
Follow up: I've resorted to sorting sets using hash() for total ordering and hash is not stable unless PYTHONHASHSEED is set. This is not really what we need but for now the one test (added here) that needs reproducibility sets it explicitly.
Needs Rec!2576 (merged)