Use packageSourceOverrides (#3)

The original reason for not using this was to
work around a mistake on our end.  When we specified
`nix` via the `extra-libraries` clause of the `.cabal`
file instead of `pkgconfig-depends` that messed up the
file generated by `cabal2nix` which entailed various
work-arounds.  Those workarounds are no longer necessary,
so we can switch to `packageSourceOverrides`

flake.nix

@@ -16,8 +16,8 @@
           haskellPackages = pkgsOld.haskellPackages.override (old: {
             overrides =
               pkgsNew.lib.fold pkgsNew.lib.composeExtensions (old.overrides or (_: _: { })) [
-                (pkgsNew.haskell.lib.packagesFromDirectory {
-                  directory = ./nix;
+                (pkgsNew.haskell.lib.packageSourceOverrides {
+                  nix-serve-ng = ./.;
                 })
                 (haskellPackagesNew: haskellPackagesOld: {
                   nix-serve-ng =

nix/nix-serve-ng.nix

@@ -1,23 +0,0 @@
-{ mkDerivation, async, base, base16, base32, bytestring, charset
-, http-client, http-types, managed, megaparsec, mtl, network, nix
-, optparse-applicative, stdenv, tasty-bench, temporary, turtle
-, vector, wai, wai-extra, warp, warp-tls
-}:
-mkDerivation {
-  pname = "nix-serve-ng";
-  version = "1.0.0";
-  src = ./..;
-  isLibrary = false;
-  isExecutable = true;
-  executableHaskellDepends = [
-    base base16 base32 bytestring charset http-types managed megaparsec
-    mtl network optparse-applicative vector wai wai-extra warp warp-tls
-  ];
-  executablePkgconfigDepends = [ nix ];
-  benchmarkHaskellDepends = [
-    async base bytestring http-client tasty-bench temporary turtle
-    vector
-  ];
-  description = "A drop-in replacement for nix-serve that's faster and more stable";
-  license = stdenv.lib.licenses.asl20;
-}