overlays: Create overlays

overlays/custom.nix

@@ -0,0 +1,5 @@
+pkgs: final: prev:
+
+import ../packages {
+  pkgs = prev;
+}

overlays/nur.nix

@@ -0,0 +1,10 @@
+pkgs: final: prev:
+
+{
+  nur = import (builtins.fetchTarball {
+    url = "https://github.com/nix-community/NUR/archive/master.tar.gz";
+    sha256 = "0r2pcc6sjxmhg1xxk23648wrwccv2c5v44cfwglx35az5y0hmr1f";
+  }) {
+    inherit pkgs;
+  };
+}

overlays/unstable.nix

@@ -0,0 +1,10 @@
+# NOTE: pkgs is unused but there for consistency with nur.nix
+pkgs: final: prev:
+
+let
+  unstableTarball =
+    fetchTarball
+      https://github.com/NixOS/nixpkgs/archive/nixos-unstable.tar.gz;
+in {
+  unstable = import unstableTarball {};
+}