a.nix

with import <nixpkgs> {};

stdenv.mkDerivation {
  name = "a";
  unpackPhase = ":";
  installPhase = ''
    mkdir $out
  '';
}

b.nix

with import <nixpkgs> {};

stdenv.mkDerivation {
  name = "b";
  unpackPhase = ":";

  propagatedBuildInputs = [
    (import ./a.nix)
  ];
  installPhase = ''
    mkdir $out
  '';
}

bprop

/nix/store/c90k7wp5785f3jsb2k07fmy37kykm9q3-a 

c.nix

with import <nixpkgs> {};

stdenv.mkDerivation {
  name = "c";
  unpackPhase = ":";

  propagatedBuildInputs = [
    (import ./b.nix)
  ];
  installPhase = ''
    mkdir $out
  '';
}

cprop

/nix/store/r38jva57s8s2rzss9ykqnm876ih2r3cp-b