Migrate Composites' zip machinery

This migrates the v2::Composites container to use the machinery introduced in LHCb!2811 (merged) and fixes the fallout.

Follow-up: remove the compatibility layer when it is no longer needed.

Goes with LHCb!2849 (merged) and Rec!2273 (merged).

cc: @ahennequ @nnolte @sstahl