Core: Fix playthrough only checking half of the sphere 0 items (#4268)

* Core: Fix playthrough only checking half of the sphere 0 items

The lists of precollected items were being mutated while iterating those
same lists, causing playthrough to skip checking half of the sphere 0
advancement items.

This patch ensures the lists are copied before they are iterated.

* Replace chain.from_iterable with two for loops for better clarity

Added a comment to `multiworld.push_precollected(item)` to explain that
it is also modifying `precollected_items`.

BaseClasses.py

@@ -1384,14 +1384,21 @@ class Spoiler:
 
         # second phase, sphere 0
         removed_precollected: List[Item] = []
-        for item in (i for i in chain.from_iterable(multiworld.precollected_items.values()) if i.advancement):
-            logging.debug('Checking if %s (Player %d) is required to beat the game.', item.name, item.player)
-            multiworld.precollected_items[item.player].remove(item)
-            multiworld.state.remove(item)
-            if not multiworld.can_beat_game():
-                multiworld.push_precollected(item)
-            else:
-                removed_precollected.append(item)
+
+        for precollected_items in multiworld.precollected_items.values():
+            # The list of items is mutated by removing one item at a time to determine if each item is required to beat
+            # the game, and re-adding that item if it was required, so a copy needs to be made before iterating.
+            for item in precollected_items.copy():
+                if not item.advancement:
+                    continue
+                logging.debug('Checking if %s (Player %d) is required to beat the game.', item.name, item.player)
+                precollected_items.remove(item)
+                multiworld.state.remove(item)
+                if not multiworld.can_beat_game():
+                    # Add the item back into `precollected_items` and collect it into `multiworld.state`.
+                    multiworld.push_precollected(item)
+                else:
+                    removed_precollected.append(item)
 
         # we are now down to just the required progress items in collection_spheres. Unfortunately
         # the previous pruning stage could potentially have made certain items dependant on others