Lazy load payment method classes

This avoids any class reloading issues in dev, simplifies most of the
code path and opens up for refactoring.

The only reason we still load the class is to call `clean_name` on it
for the translation key. We can probably do better.

app/controllers/spree/admin/payment_methods_controller.rb

@@ -14,7 +14,7 @@ module Spree
         Spree::PaymentMethod::Check
         Spree::Gateway::PayPalExpress
         Spree::Gateway::StripeSCA
-      }.index_with(&:constantize).freeze
+      }.freeze
 
       def create
         force_environment
@@ -117,7 +117,7 @@ module Spree
       end
 
       def validate_payment_method_provider
-        valid_payment_methods = PAYMENT_METHODS.keys
+        valid_payment_methods = PAYMENT_METHODS
         return if valid_payment_methods.include?(params[:payment_method][:type])
 
         flash[:error] = Spree.t(:invalid_payment_provider)
@@ -133,13 +133,13 @@ module Spree
       end
 
       def load_providers
-        providers = PAYMENT_METHODS.values.sort_by(&:name)
+        providers = PAYMENT_METHODS.sort
 
         unless show_stripe?
           providers.reject! { |provider| stripe_provider?(provider) }
         end
 
-        providers
+        providers.map(&:constantize)
       end
 
       # Show Stripe as an option if enabled, or if the
@@ -165,7 +165,7 @@ module Spree
       end
 
       def stripe_provider?(provider)
-        provider.name.ends_with?("StripeSCA")
+        provider.ends_with?("StripeSCA")
       end
 
       def base_params