B00_ocaml.OcamlpathSourceOCAMLPATH search path.
FIXME, maybe this should be a store key.
get m o k is k ps if o is Some ps and otherwise in order:
OCAMLPATH environment variable is defined in m and non-empty its content is parsed s according to B0_std.Fpath.list_of_search_path.OPAM_SWITCH_PREFIX environment variable is defined with a path P then [P/lib] is used.