fix in persistreco config

Fix in persistreco config.

Commit 4964bedb was added to fix a test in !4295 (merged), while it causes problems with configuring PbPb persistreco properly. Function https://gitlab.cern.ch/lhcb/LHCb/-/blob/master/PyConf/python/PyConf/packing.py#L62 is used for multiple purposes it can't do both (e.g. reading location on one hand, configuring persistreco on the other).

More general problem, reading/writing (incl. reco python config (reco from file...)) versus persistreco version is entangled and causes problems in logic. Clearer separation is needed. Leftover from design change from persistable_location_version to persistreco_version (Moore!2338 (merged) and others).

fyi: @rmatev

goes with Moore!2732 (merged)