Properly eager load the JoinDependency autoloads

After rails#43859, the constants were being registered to be eagear loaded
but the eager load was not being called on it, so they were still being
required at runtime.

activerecord/lib/active_record/associations.rb

@@ -290,6 +290,7 @@ module Builder # :nodoc:
     def self.eager_load!
       super
       Preloader.eager_load!
+      JoinDependency.eager_load!
     end
 
     # Returns the association instance for the given name, instantiating it if it doesn't already exist