fix startup crash when auto-enabling plugin-provided datapacks

[wiselder]

It checks the identifiers of the selected packs, rather than comparing the pack objects when selecting them again.

This allows you to avoid converting Adventure pack names before the Minecraft registries become available, which could lead to a NullPointerException when starting the server.

crash log

Local testing:

1. Paper 26.2
2. One Paper plugin with Bootstrap and a built-in datapack.
3. Register it twice in DATAPACK_DISCOVERY under different identifiers and specify autoEnableOnServerStart(true) for both.
4. Start the server in an existing world.
   Result: NPE in AdventureComponent.equals() -> PackRepository.rebuildSelected(), the server does not start.
   The error was not reproduced when autoEnableOnServerStart(false).

[EggAllocationService]

+1, this issue is causing problems for me as it can also occur when two different plugins load their own built in datapacks during bootstrap.