Fix temporary class definition

Before this change, the perform method was using the codebase's
PackageFile class directly.

lib/gitlab/background_migration/set_null_package_files_file_store_to_local_value.rb

@@ -11,10 +11,12 @@ module Gitlab
     class SetNullPackageFilesFileStoreToLocalValue
       LOCAL_STORE = 1 # equal to ObjectStorage::Store::LOCAL
 
+      module Packages
         # Temporary AR class for package files
         class PackageFile < ActiveRecord::Base
           self.table_name = 'packages_package_files'
         end
+      end
 
       def perform(start_id, stop_id)
         Packages::PackageFile.where(file_store: nil, id: start_id..stop_id).update_all(file_store: LOCAL_STORE)