fix: Update namespace of api v1 legacy

[JuanseGimenez]

WHEN

When use spree_multi_vendor with spree_api_v1 base_controller namspace is broken, and load unnecessary files and broken when do not use spree_api_v1

CHANGES

• add api_v1_available? method to engine
• change namespace on base_controller_decorator.rb
• change namespace on products_controller_decorator.rb
• change namespace on users_controller_decorator.rb
• change namespace base on vendors_controller.rb
• add conditional to load api_helpers_decorator.rb

[rafalcymerys]

@nciemniak could you take a look at this PR? I remember you made similar changes in spree_auth_devise