Enhance Derived Entries to link to Source

[mlkirkpatrick]

deriveLinkedEntries (and its close cousin, transformEntriesToType) provides an option to update the source item with a reference to the newly derived entry. I need the reference to go the other direction: target -> source.

Seems like this should be simple, but there doesn't seem to be a way to reference the source entry id during a migration.

[marcolink]

Hi @mlkirkpatrick 👋
We have just added the entry id to all transform callbacks
#1212