Follow-up from "Resolve "Optimize Routable.where_full_path_in by avoiding joins""

The following discussion from !137886 (merged) should be addressed:

• @tkuah started a discussion:

          source_type_condition = { source_type: route_scope.klass.base_class }
          routes_matching_condition = Route.where(source_type_condition).where(wheres.join(' OR '))
          source_ids = routes_matching_condition.pluck(:source_id)
  
          route_scope = all
          result = route_scope.where(id: source_ids)

  Suggestion:, minor restructure to group related variables together.