Fixes actor stacking bug - UnloadCargo and Cargo.SpawnPassengers

Supersedes #6060 and #6063
Fixes UnloadCargo and Cargo.SpawnPassengers by queuing a slightly smarter nudge w/ others that
have the same CenterPosition.
Because this code is interested primarily in the cell (sub-)grid you should be checking against `var other in self.World.ActorMap.GetUnitsAt(cell, toSubCell).Where(a => a != self)`.  `CenterPosition` will currently give you the right result, but that is not guaranteed to remain true in the future (different actors may want to define different offsets for the same subcell).