An incomplete iTunes backup still lists in its Manifest.db the files it
failed to acquire, so a module which found nothing for one of them looks
exactly like a module which found nothing on a device that never had the
artifact. The Manifest module now records a "missing" flag on those
records and reports the total, so a gap in the acquisition is visible in
manifest.json and in the command output.
The stored file IDs are collected by walking the backup folder once, which
keeps the check off the per-entry filesystem lookup path: on a 20k entry
manifest the module runs in the same time as before the change.