Include product UPC as original item id, for Catapult export

in case data needs to be imported back into CORE, from spreadsheet

rattail_corepos/corepos/importing/db/exporters/catapult_inventory.py

@@ -609,6 +609,7 @@ class InventoryItemImporter(FromCore, catapult_importing.model.InventoryItemImpo
 
         return {
             '__product__': product,
+            '__original_item_id__': product.upc,
             'uuid': get_uuid(),
             'item_id': item_id,
             'dept_id': department.number if department else None,