opam: ensure dependents don't have to depend on ppx_tools directly.
opam: ensure dependents don't have to depend on ppx_tools directly.

rneswold/lwt,mfp/lwt,c-cube/lwt,rgrinberg/lwt,rgrinberg/lwt,rgrinberg/lwt,mirage/lwt,ocsigen/lwt,c-cube/lwt,c-cube/lwt,rgrinberg/lwt,fdopen/lwt,rneswold/lwt,ocsigen/lwt,mfp/lwt,mirage/lwt,rneswold/lwt,fdopen/lwt,ocsigen/lwt