Avoid name clashing when including modules

Rails' autoloading is a bit strange. It relies on `const_missing`, which is run when an undefined constant is referenced. If there is a `::Checkout` constant already defined (which would happen if it had been referenced already before this file had been loaded), it would be a valid constant for ruby to find here and `const_missing` won't be triggered.

Most won't need this change, but I don't think there's harm in including it.