Fix variant eager loading in Shipment `manifest`

See https://github.com/spree/spree/pull/3687
Allow users to dont split packages

Replay changes from https://github.com/spree/spree/pull/3618
Fix order_id assignment in OrderInventory

See https://github.com/spree/spree/pull/3712

Conflicts:
	app/models/spree/order_inventory_assembly.rb
	app/models/spree/order_inventory_decorator.rb