OCAML_NAMESPACES shadowing order

[dbuenzli]

Is currently defined as last one takes over:

Modules introduced by later namespaces in OCAML_NAMESPACES will shadow modules introduced by earlier namespaces. In addition, modules introduced by -I or -P will shadow modules introduced via OCAML_NAMESPACES.

however it is customary for PATH variables to have the first one takes over.