Simplification/Streamlining of GaudiConf.reading

There are only two functions in GaudiConf.reading which are (need to be) exported: do_unpacking and type_map. A few other functions are only invoked by reco_objects_for_spruce.py, and are moved there. do_unpacking is a special case (it configures unpacking using configurables for GaudiPython, bypassing PyConf) and it is only used by qmtests of the sprucing. This MR streamlines and simplifies do_unpacking so that it exactly meets the requirements of that use case, and removes any suggestion of it being more general, which it actually isn't.

Must be applied in conjunction with Moore!2983 (merged)