Implementations may refer to virtual library's modules
  $ dune build
  bar from vlib
  Foo.run implemented
  $ find _build -iname "*.ml-gen" | sort | while read file; do echo "contents of $file"; cat $file; echo "--------"; done;
  contents of _build/default/impl/vlib__impl__.ml-gen
  (* generated by dune *)
  
  (** @canonical Vlib.Bar *)
  module Bar = Vlib__Bar
  
  (** @canonical Vlib.Foo *)
  module Foo = Vlib__Foo
  --------
  contents of _build/default/vlib/vlib.ml-gen
  (* generated by dune *)
  
  (** @canonical Vlib.Bar *)
  module Bar = Vlib__Bar
  
  (** @canonical Vlib.Foo *)
  module Foo = Vlib__Foo
  --------
