Generalized data input iterator

To simplify command block generation (in particular for automated tool generation) it would be nice to have a function to iterate data sets, composite data sets, collections, and all sorts of collections. Ideally this would give access to the path to the data set and the identifier.

The goal would be to iterate over a data input without the need to know what it actually is.

No files, tests, or entry points are named. Start by locating the existing command block generation and data input representations, then compare how datasets, composite datasets, and collections are handled. Done means a single iteration approach can expose each input's path and identifier without requiring callers to know its concrete type.

Written by the indexing model from the issue text.