Fix autoloading of DisableJoins constants

These are prepended onto ActiveRecord classes which are not autoloaded.
This means autoloading does not work well inside these classes because
some references could be stale when autoloaded classes are unloaded.

lib/gem_extensions/active_record/association.rb

@@ -15,7 +15,7 @@ module GemExtensions
 
       def scope
         if disable_joins
-          DisableJoins::Associations::AssociationScope.create.scope(self)
+          ::GemExtensions::ActiveRecord::DisableJoins::Associations::AssociationScope.create.scope(self)
         else
           super
         end
@@ -25,7 +25,7 @@ module GemExtensions
         if klass
           @association_scope ||= begin # rubocop:disable Gitlab/ModuleWithInstanceVariables
             if disable_joins
-              DisableJoins::Associations::AssociationScope.scope(self)
+              ::GemExtensions::ActiveRecord::DisableJoins::Associations::AssociationScope.scope(self)
             else
               super
             end

lib/gem_extensions/active_record/disable_joins/associations/association_scope.rb

@@ -64,7 +64,7 @@ module GemExtensions
             end
 
             if scope.order_values.empty? && ordered
-              split_scope = DisableJoins::Relation.create(scope.klass, key, join_ids)
+              split_scope = ::GemExtensions::ActiveRecord::DisableJoins::Relation.create(scope.klass, key, join_ids)
               split_scope.where_clause += scope.where_clause
               split_scope
             else