Allow sideloading to check for uniqueness using other identifiers than id

To check if a sideloaded resource already exists in the global scope, it should be allowed to check using something else than `id`

Start by tracing the sideloading flow and the global-scope existence check described in the issue. Determine how uniqueness currently depends on id and define how an alternate identifier should be supplied and verified; done means sideloaded resources can be matched using a non-id identifier without breaking id-based checks.

Written by the indexing model from the issue text.