Convert taxonomy to entity reference
I don't have time to fully flesh it out at the moment, but the approach I would take would be to:


In your product migration, which presumably is using the d7_node source plugin, override that source plugin and implement prepareRow() (not forgetting to call parent::prepareRow()!) to query the related ingredient terms and $row->setSourceProperty('ingredient_names', /* array of term names *?);
Use the migrate_plus entity_lookup process plugin to map those ingredient_names to existing (previously imported) ingredient node titles. You'll be looking at the example with full configuration for your use case...


Hope this helps!