Fixed UnloadCargo stacking using new subcell API exposure

Partially supersedes #6083, Cargo.SpawnPassengers will have to wait for my Exit refactoring and discussion on how unloading while being sold should be handled.
Naming: `Subcell` &rarr; `SubCell`.

I kind of regret using a capital C when first introducing subcells, but we can fix that if/when we merge the subcell index into Cell.