add sample hello package (copied from nixpkgs)

packages/hello/default.nix

@@ -0,0 +1,68 @@
+{
+    # Snowfall Lib provides a customized `lib` instance with access to your flake's library
+    # as well as the libraries available from your flake's inputs.
+    lib,
+    # You also have access to your flake's inputs.
+    inputs,
+
+    # The namespace used for your flake, defaulting to "internal" if not set.
+    namespace,
+
+    # All other arguments come from NixPkgs. You can use `pkgs` to pull packages or helpers
+    # programmatically or you may add the named attributes as arguments here.
+    stdenv,
+    fetchurl,
+    nixos,
+    testers,
+    versionCheckHook,
+    hello,
+    ...
+}:
+stdenv.mkDerivation (finalAttrs: {
+  pname = "hello";
+  version = "2.12.1";
+
+  src = fetchurl {
+    url = "mirror://gnu/hello/hello-${finalAttrs.version}.tar.gz";
+    hash = "sha256-jZkUKv2SV28wsM18tCqNxoCZmLxdYH2Idh9RLibH2yA=";
+  };
+
+  # The GNU Hello `configure` script detects how to link libiconv but fails to actually make use of that.
+  # Unfortunately, this cannot be a patch to `Makefile.am` because `autoreconfHook` causes a gettext
+  # infrastructure mismatch error when trying to build `hello`.
+  env = lib.optionalAttrs stdenv.hostPlatform.isDarwin {
+    NIX_LDFLAGS = "-liconv";
+  };
+
+  doCheck = true;
+
+  doInstallCheck = true;
+  nativeInstallCheckInputs = [
+    versionCheckHook
+  ];
+
+  # Give hello some install checks for testing purpose.
+  postInstallCheck = ''
+    stat "''${!outputBin}/bin/${finalAttrs.meta.mainProgram}"
+  '';
+
+  passthru.tests = {
+    version = testers.testVersion { package = hello; };
+  };
+
+  passthru.tests.run = callPackage ./test.nix { hello = finalAttrs.finalPackage; };
+
+  meta = {
+    description = "Program that produces a familiar, friendly greeting";
+    longDescription = ''
+      GNU Hello is a program that prints "Hello, world!" when you run it.
+      It is fully customizable.
+    '';
+    homepage = "https://www.gnu.org/software/hello/manual/";
+    changelog = "https://git.savannah.gnu.org/cgit/hello.git/plain/NEWS?h=v${finalAttrs.version}";
+    license = lib.licenses.gpl3Plus;
+    maintainers = with lib.maintainers; [ stv0g ];
+    mainProgram = "hello";
+    platforms = lib.platforms.all;
+  };
+})