pkgs/overlay: create selfExpr

adds a package that references nixpkgs
config of nixpkgs that created it is retained

pkgs/overlays/selfExpr.nix

@@ -0,0 +1,21 @@
+{ nixpkgsPath ? <nixpkgs> }:
+
+self: super: {
+  selfExpr = let
+    config = builtins.removeAttrs self.config [ "_undeclared" ];
+    configJson = builtins.toJSON config;
+  in
+    self.writeTextFile {
+    name = "nixpkgs-self";
+    destination = "/default.nix";
+    text = ''
+      { ... } @ args:
+
+      import ${builtins.toString nixpkgsPath} {
+        config = (builtins.fromJSON '''
+          ${configJson}
+        ''') // args.config or {};
+      } // builtins.removeAttrs args [ "config" ]
+    '';
+  };
+}