WIP: move order models under the order namespace.

There are several models that belong under the Spree::Order namespace whose sole responsibilities are related to interacting with an order. Because of this I believe they should be moved under the Spree::Order namespace.

This is currently a WIP

- [x] Move OrderPopulator to Spree::Order::Populator
- [ ] Move OrderInventory to Spree::Order::Inventory
- [ ] Move OrderUpdater to Spree::Order::Updater
- [ ] Move OrderContents to Spree::Order::Contents