shop cleanup and correctly backreference swapped items' locations

Also fixes a false reference in progression balancing from 2019 (swapped Location.item.location was not updated)

Fill.py

@@ -1,7 +1,7 @@
 import logging
 import typing
 
-from BaseClasses import CollectionState, PlandoItem
+from BaseClasses import CollectionState, PlandoItem, Location
 from Items import ItemFactory
 from Regions import key_drop_data
 
@@ -336,7 +336,8 @@ def balance_multiworld_progression(world):
                             replacement_locations.insert(0, new_location)
                             new_location = replacement_locations.pop()
 
-                        new_location.item, old_location.item = old_location.item, new_location.item
+                        swap_location_item(old_location, new_location)
+
                         new_location.event, old_location.event = True, False
                         logging.debug(f"Progression balancing moved {new_location.item} to {new_location}, "
                                       f"displacing {old_location.item} in {old_location}")
@@ -361,6 +362,18 @@ def balance_multiworld_progression(world):
                 raise RuntimeError('Not all required items reachable. Something went terribly wrong here.')
 
 
+def swap_location_item(location_1: Location, location_2: Location, check_locked=True):
+    """Swaps Items of locations. Does NOT swap flags like event, shop_slot or locked"""
+    if check_locked:
+        if location_1.locked:
+            logging.warning(f"Swapping {location_1}, which is marked as locked.")
+        if location_2.locked:
+            logging.warning(f"Swapping {location_2}, which is marked as locked.")
+    location_2.item, location_1.item = location_1.item, location_2.item
+    location_1.item.location = location_1
+    location_2.item.location = location_2
+
+
 def distribute_planned(world):
     world_name_lookup = {world.player_names[player_id][0]: player_id for player_id in world.player_ids}
 
@@ -368,7 +381,8 @@ def distribute_planned(world):
         placement: PlandoItem
         for placement in world.plando_items[player]:
             if placement.location in key_drop_data:
-                placement.warn(f"Can't place '{placement.item}' at '{placement.location}', as key drop shuffle locations are not supported yet.")
+                placement.warn(
+                    f"Can't place '{placement.item}' at '{placement.location}', as key drop shuffle locations are not supported yet.")
                 continue
             item = ItemFactory(placement.item, player)
             target_world: int = placement.world