Ref T2787. Carts need a status so we can tell if they've been purchased. Also kind of get WePay working as a one-time provider, and let charges not have a methodPHID (they won't for one-time providers).
All the status stuff is still super crazy rough and you can do things like start a checkout, add a bunch of stuff to your cart, complete the checkout, and have Phabricator think you paid for all the stuff you added. But this is fine for now since you can't actually edit carts, and also none of this is at all usable anyway. I'll refine some of the workflows in future diffs, for now I'm just getting things hooked up and technically working.