ocamlbuild: make -plugin-tags and -use-ocamlfind interact better

When both those features are used together, we use ocamlfind packages
instead of hardlinking unix.cmxa and ocamlbuildlib.cmxa during the
ocamlbuild plugin compilation. This helps to avoid double-linking
errors (with unix.cmxa and "-package unix") when -plugin-tags request
ocamlfind packages depending on unix (or ocamlbuild).

This change was designed and implemented in collaboration with Jun Maillard.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14069 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02