Fix for https://github.com/cms-sw/cmssw/pull/22594

Instead of assigning rule that do not correspond to any specific branches
(for example rules setting transients or whole objects) to an hypothetical
'previous' branch, we switch to assign those orphan rules to either the
non-data bearing branches (split single objects or bases) or the collection
node (since the splitting completely flatten the hiearchy in this case).

This requires enhancing the list of IDs from a single list of element index
in the 'sole' current StreamerInfo to a nested list of elements that carries
along the sub-StreamerInfo information.