Avoid name clashing when including modules

We should probably have a `require 'spree/order/checkout'` at the top here. We're depending on Rails autoloading to load the correct constant, but if `Checkout` is loaded before this point then it will reference that constant, which is wrong. The require would ensure that the right constant is loaded and it would always mean `Spree::Order::Checkout`, and not the top level `Checkout`.