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 =