GENERIC_PACK_STRIP_DIRS is not working

### Describe the problem

I got a .zip modpack where the folder structure is _/modpack folder name/mods folder_ but when I load the zip with

```
- GENERIC_PACK=/modpacks/modpack.zip
- GENERIC_PACK_STRIP_DIRS=1
```
the server starts creating the game data but the mods folder is empty. If I repeat the process manually editing out the modpack folder name from the zip leaving the mods folder and the other files directly in to the zip root it works

### Container definition

```
Paste run command or compose file here
```
minecraft:
[...]
environment:
- MAX_MEMORY=8G
- EULA=TRUE
- ENABLE_RCON=false
- MAX_PLAYERS=8
- OPS=_redacted_
- VIEW_DISTANCE=15
- ALLOW_FLIGHT=true
- MAX_TICK_TIME=-1
- TYPE=FABRIC
- VERSION=1.18.1
- GENERIC_PACK=modpack.zip
- GENERIC_PACK_STRIP_DIRS=1
volumes:
- ./containers/games/minecraft/data:/data
- ./containers/games/minecraft/modpacks:/modpacks:ro
[...]

### Container logs

```
Paste logs here
```

Start by tracing the startup handling for GENERIC_PACK and GENERIC_PACK_STRIP_DIRS in the container. Reproduce the reported archive layout with a Fabric 1.18.1 server, compare it with an archive whose contents are at the root, and verify that the mods directory is populated in both cases.

Written by the indexing model from the issue text.