This reintroduces formation-like behaviour in the form of "grouped walking", ensuring that units walk together in a mostly sane manner.
Unlike our current formation code, it's handled strictly inside of UnitAI and doesn't require any change elsewhere. It uses a group-walk-manager system component to keep track of some things.
The result isn't nearly as neat, but units no longer bypass each other's obstruction (which is in large part why it's less neat). With pushing, the results would most likely be much better. It can also be improved quite a lot.