fix item_contents save/load

[KorGgenT]

Summary

SUMMARY: Bugfixes "fix item_contents save/load"

Purpose of change

related #40127 - possibly fixes it but i haven't checked random item spawns as this doesn't effect them at all

Fixes #40876

Describe the solution

the load function for item_contents incorrectly assumes that the insert_item function will insert the item into the same pocket that the item was saved in - but this is definitely not the case as items are dynamically assigned and possibly specifically assigned by the player. additionall, the MIGRATION item pocket was never utilized such that items won't disappear on save/load.

a weakness of this implementation is the following example: an item has multiple pockets, and someone changes the json to rearrange the pockets, or add a MAGAZINE pocket to the front. in this case, most of the items will be moved to the MIGRATION pocket, necessitating the player to pick up the items after they fall to the ground after moving once.

Testing

several save/load cycles, with 2 army jackets and jeans and assorted sundry of items. also spawned in canned sardines to make sure they stay sealed across the save/load cycle.

Additional context

this might be one of the main things that are causing cans to appear to spawn open, maybe people are saving and loading near enough to houses to have the items spawned and go through the save/load cycle. it's worth reassessing the related bug report after this bugfix.