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.
I'd prefer that we didn't add bandaids like this, as they hurt code maintainability and don't fix the underlying problem.  We should identify and fix the situations where units move into an already-occupied subcell.