The inventory was being cleared by removing the entity
from the world, but Folia changed the remove to be
before the entity copy which caused the new entity
to have a cleared inventory. Fix this by creating
a post-dimension copy callback that will clear
the inventory of the old entity.
Fixes https://github.com/PaperMC/Folia/issues/29
The code to stop all brain tasks is required to pass the current
game time to the tasks it stops. But, when a villager is being
portalled, the copied entity does not have any running tasks. So,
we can simply return early before invoking getGameTime if there
are no running tasks.
Fixes https://github.com/PaperMC/Folia/issues/23
