autoindex: stop tracking deletions, use generic autofile tracking instead

autoindex: stop tracking deletions, use generic autofile tracking instead

- Migrate the set of deletions to the {autofile} set, since it has
  more or less the same effect. This affects the "deleted" case in the
  test.
- If a page has just been deleted, add it as an autofile anyway: by
  the time gen_autofile is called, it'll be in the list of deleted files,
  so it'll just be added to {autofile}. This affects the "gone" case
  in the test.
- Behaviour change: we don't forget that a page with no reason to be
  re-created was deleted. This affects the 'expunged' and 'reinstated'
  cases in the test.