CLONE - Backup and restore: External files

Test Steps:
1. Create a course, and add a reference file.
2. Backup this course, at the end of backup process, you should be able to see a warning, "There are file references in this course, they can only be restored in this site"
3. Save the backup file
4. Restore the course on another moodle site (using the saved backup file), at the end of restoring process, you will warned some files cannot be restored because they are references
5. Restore the course on the original site. There will not be any warnings and all file references will be restored correctly.