Translate items / asset to id in single import script

When importing, assets / items are referenced by external_id if they do not exist in target environment.

However, if these items / assets were created in the same import, they can be just referenced with their new id instead. This way we wouldn't need to use external_id at all.

Start by tracing the single-import flow that resolves asset and item references; the issue does not name a file, entry point, or test. Confirm how newly created entities receive IDs and how external_id references are currently handled. Done means same-import entities use their new IDs, while entities absent from the target still resolve through external_id.

Written by the indexing model from the issue text.