Add package import magic

pkgs/default.nix

@@ -0,0 +1,5 @@
+import <nixpkgs> {
+  overlays = [
+    (import ./overlay.nix)
+  ];
+}

pkgs/overlay.nix

@@ -0,0 +1,3 @@
+self: super: {
+  anycast_healthchecker = self.python3.pkgs.callPackage ./anycast_healthchecker {};
+}