use a strengthening version of scrape_alias in Typemod

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/module-alias@14279 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
forgot test...

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/module-alias@14280 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02