Move order related models to 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.

- [x] Move OrderPopulator to Spree::Order::Populator
- [x] Move OrderInventory to Spree::Order::Inventory
- [x] Move OrderUpdater to Spree::Order::Updater
- [x] Move OrderContents to Spree::Order::Contents