Fix some problems with code that was relying on spree code

app/models/spree/payment_method.rb

@@ -99,7 +99,7 @@ module Spree
         self.class.include Spree::Core::CalculatedAdjustments
       end
 
-      self.calculator ||= Calculator::FlatRate.new(preferred_amount: 0)
+      self.calculator ||= ::Calculator::FlatRate.new(preferred_amount: 0)
     end
 
     def has_distributor?(distributor)

spec/factories/calculator_factory.rb

@@ -1,9 +1,9 @@
 FactoryBot.define do
-  factory :calculator, class: Spree::Calculator::FlatRate do
+  factory :calculator, class: Calculator::FlatRate do
     after(:create) { |c| c.set_preference(:amount, 10.0) }
   end
 
-  factory :no_amount_calculator, class: Spree::Calculator::FlatRate do
+  factory :no_amount_calculator, class: Calculator::FlatRate do
     after(:create) { |c| c.set_preference(:amount, 0) }
   end
 

spec/factories/stock_factory.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 FactoryBot.define do
-  factory :stock_package, class: Spree::Stock::Package do
+  factory :stock_package, class: OrderManagement::Stock::Package do
     ignore do
       stock_location { build(:stock_location) }
       order { create(:order_with_line_items, line_items_count: 2) }