TUNIC: Deal with other games placing stuff in other worlds during pre_fill

[ScipioWright]

What is this fixing or adding?

Yes yes, I know, you're not supposed to place stuff in other worlds during pre_fill (presumably, idk if the docs even say that anywhere). But, we've got like 300 non-verified apworlds around. So I'd rather just be more resilient than pout and say "but muh spec".

During the local filler stuff that Tunic does for grass and breakable shuffle, during pre_fill it'll grab a set of locations to fill during stage_pre_fill. If another world fills any of those locations between that Tunic slot's pre_fill and stage_pre_fill, it'll crash when doing place_locked_item. So, this will make it more resilient to that sort of crash by placing it in the item pool. I know we're not supposed to place stuff in the item pool during pre_fill, but you also aren't supposed to fill other worlds' locations during pre_fill. It's something that is out of spec that only happens if something else is out of spec, so I'd say that's fair game.

How was this tested?

LlisandurFF4.txt
Ixrec Tunic.txt

Test gens with FF4FE. Yamls are attached (converted to .txt since github doesn't let you just upload .yamls).

[Rosalie-A]

Tested this with a world that places items offworld in pre_fill, and did not encounter any issues in a dozen generations.

[Exempt-Medic]

This violates expectations and the test_itempool_not_modified unit test. This should not be merged

[Exempt-Medic]

https://github.com/ArchipelagoMW/Archipelago/blob/11130037fe9a88d8be70a1624c35aea17daab680/docs/world%20api.md#generation

create_items(self) called to place player's items into the MultiWorld's itempool. By the end of this step all regions, locations and items have to be in the MultiWorld's regions and itempool. You cannot add or remove items, locations, or regions after this step. Locations cannot be moved to different regions after this step. This includes event items and locations.