Due to the package structure and location of MASpawnThread, SpawnsPets doesn't need to be opened up for this to work.
This change opens up for quite a few different upgrade strategies, since pet items obtained outside of Upgrade Waves are transformed as well. It is technically a breaking change, because it breaks the invariant that "after arena start, no items are transformed to pets". However, since pet items are customizable, it is possible to just change the `bone` to `sponge` in the config-file if bones carry some sort of implicit meaning in an existing setup.
Closes#524