merge patch for PR#6465 (allow incremental weakening of module aliases

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15025 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
revert commit 15025 (PR#6465) as it breaks camlp4

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15026 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02