testlib.nix

with import <nixpkgs> {};
with stdenv;

let
  a = runCommand "a" {} "mkdir -p $out/lib/mydir; touch $out/lib/mydir/a";
  b = runCommand "b" {} "mkdir -p $out/lib/mydir; touch $out/lib/mydir/b";
in
runCommand "c" { buildInputs = [ a b ]; } ''
  echo "Where can I find lib/mydir...?"
''

with import <nixpkgs> {};
with stdenv;

let
  a = runCommand "a" {} "mkdir -p $out; touch $out/a";
  b = runCommand "b" {} "mkdir -p $out; touch $out/b";
  c = runCommand "b" { inputs = [a b]; } ''
    mkdir $out
    for i in $inputs; do
      for f in $i/*; do
        ln -s $f $out/
      done
    done
  '';
in
runCommand "z" { input = c; } ''
  echo "All inputs:"
  ls -l $input
''