Rebuild the Checkout class, no discounts yet

Checkout now stores its items using a quantity and a link back to the
original product instance. It has a reference to the store to make sure
that product IDs added are valid

shop.rb

@@ -9,14 +9,12 @@ class Shop
   end
 
   def add_product(product)
-    raise DuplicateProductIdError if duplicate_id?(product.id)
+    raise DuplicateProductIdError if product_by_id(product.id)
 
     @products.push(product)
   end
 
-  private
-
-  def duplicate_id?(id)
-    @products.map(&:id).include?(id)
+  def product_by_id(id)
+    @products.find { |product| product.id == id }
   end
 end