Private packages not found in public libraries using private libraries

[toots]

Here's a gist to demonstrate the issue: https://gist.github.com/toots/aae9382977ceb6661f0dac66ee01aa53

To reproduce:

% dune build @install
% opam install -y .

Then, in a separate directory:

% cat test.ml
let () =
  Printf.printf "Public value: %s\n%!" Test_private_lib.Public.public_value

% ocamlfind ocamlopt -linkpkg -package test_private_lib -o test test.ml
ocamlfind: Package `private' not found - required by `test_private_lib'