Can't migrate a deleted gun variant

[hexagonrecursion]

Describe the bug

#72398 removed a gun variant. This caused errors when loading saves (see #72602). It looks like #50266 was supposed to allow migrating specific item variants, but this did not work when I tried it. Through trial and error I have narrowed this down: a variant can only be migrated it is has not been deleted yet.

Attach save file

N/A

Steps to reproduce

1. Install this mod:

   [
     {
       "type": "MOD_INFO",
       "id": "variant_migrations",
       "name": "variant_migrations",
       "authors": [ "Your name here" ],
       "description": "variant_migrations",
       "category": "content",
       "dependencies": [ "dda" ]
     },
     {
       "id": "foo_bar_common",
       "type": "GUN",
       "volume": "1000 ml",
       "weight": "1000 g",
       "variant_type": "gun",
       "name": { "str": "name" },
       "description": "description",
       "symbol": "B",
       "skill": "pistol"
     },
     {
       "id": "foo",
       "type": "GUN",
       "copy-from": "foo_bar_common",
       "name": { "str": "foo" },
       "variants": [
         { "id": "v1", "name": { "str": "foo v1" }, "description": "description" },
         { "id": "v2", "name": { "str": "foo v2" }, "description": "description" },
         { "id": "v3", "name": { "str": "foo v3" }, "description": "description" }
       ]
     }
   ]

2. Spawn all 3 variants of foo

3. Save, quit and replace the mod with:

   [
     {
       "type": "MOD_INFO",
       "id": "variant_migrations",
       "name": "variant_migrations",
       "authors": [ "Your name here" ],
       "description": "variant_migrations",
       "category": "content",
       "dependencies": [ "dda" ]
     },
     {
       "id": "foo_bar_common",
       "type": "GUN",
       "volume": "1000 ml",
       "weight": "1000 g",
       "variant_type": "gun",
       "name": { "str": "name" },
       "description": "description",
       "symbol": "B",
       "skill": "pistol"
     },
     {
       "id": "foo",
       "type": "GUN",
       "copy-from": "foo_bar_common",
       "name": { "str": "foo" },
       "variants": [
         { "id": "v1", "name": { "str": "foo v1" }, "description": "description" },
         { "id": "v2", "name": { "str": "foo v2" }, "description": "description" }
       ]
     },
     {
       "id": "bar",
       "type": "GUN",
       "copy-from": "foo_bar_common",
       "name": { "str": "bar" },
       "variants": [
         { "id": "v2", "name": { "str": "bar v2" }, "description": "description" },
         { "id": "v3", "name": { "str": "bar v3" }, "description": "description" }
       ]
     },
     {
       "type": "MIGRATION",
       "id": "foo",
       "from_variant": "v2",
       "replace": "bar"
     },
     {
       "type": "MIGRATION",
       "id": "foo",
       "from_variant": "v3",
       "replace": "bar"
     }
   ]

4. When loading you get an error:

   DEBUG : item 'foo' has no variant 'v3'!

   FUNCTION : void item::set_itype_variant(const string&)
   FILE : src/item.cpp
   LINE : 9336
   VERSION : cdda-experimental-2024-03-25-0528 74771e1

5. foo v2 is correctly migrated to bar v2, but foo v3 is replaced with a different variant of foo instead of being migrated to bar v3

Expected behavior

1. No error
2. foo v3 is migrated to bar v3

Screenshots

No response

Versions and configuration

• OS: Linux
  • OS Version:
• Game Version: cdda-experimental-2024-03-25-0528 74771e1 [64-bit]
• Graphics Version: Tiles
• Game Language: System language []
• Mods loaded: [
  Dark Days Ahead [dda],
  Disable NPC Needs [no_npc_food],
  Portal Storms Ignore NPCs [personal_portal_storms],
  Slowdown Fungal Growth [no_fungal_growth],
  variant_migrations [variant_migrations]
  ]

Additional context

Pinging @anothersimulacrum because variant migrations were originally added in #50266

[anothersimulacrum]

Thank you for the excellent reproduction steps!