a.nix

{ stdenv, smartmontools, pciutils }:

stdenv.mkDerivation {
  name = "hello-a";
  src = ./hello-2.9.tar.gz;
  buildInputs = [ smartmontools ];
  propagatedBuildInputs = [ pciutils ];
}

b.nix

{ stdenv, a }:

stdenv.mkDerivation {
  name = "hello-b";
  src = ./hello-2.9.tar.gz;
  buildInputs = [ a hdparm ];
}

c.nix

{ stdenv, b }:

stdenv.mkDerivation {
  name = "hello-c";
  src = ./hello-2.9.tar.gz;
  buildInputs = [ b ];
}

default.nix

with import <nixpkgs> {};
rec {
  a = callPackage ./a.nix { };
  b = callPackage ./b.nix { inherit a; };
  c = callPackage ./c.nix { inherit b; };
}