make nix overlay and remove nixpkgs dependency

flake.nix

@@ -1,59 +1,54 @@
 {
   description = "Add support for deserialization of pydantic child models";
 
-  inputs = {
-    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
-  };
-
-  outputs = { self, nixpkgs }: let
+  outputs = { self }: let
     system = "x86_64-linux";
-    pkgs = import nixpkgs { inherit system; };
     packageName = "pydantic-uuid-model";
     version = "1.1.0";
-
-    pydantic = (ps: ps.buildPythonPackage rec {
-      pname = "pydantic";
-      version = "1.9.1";
-      src = ps.fetchPypi {
-        inherit pname version;
-        sha256 = "sha256-HtmHw/8p//f9jD6jo+qHetMQquLvmImhGeItPy2waRo=";
-      };
-    });
   in {
 
-    packages."${system}" = {
-      "${packageName}" = pkgs.python3Packages.callPackage ./default.nix {
-        inherit packageName version;
-	pydantic = (pydantic pkgs.python3Packages);
-      };
-
-      default = self.packages."${system}"."${packageName}";
-
-      overlay = (final: prev: {
-        pythonPackagesExtensions = prev.pythonPackagesExtensions ++ [
+    overlays = let
+      composeExtensions = (
+        f: g: final: prev:
+        let
+          fApplied = f final prev;
+          prev' = prev // fApplied;
+        in
+        fApplied // g final prev');
+    in {
+      default = builtins.foldl' composeExtensions (final: prev: {}) [
+	self.overlays.pydantic
+        self.overlays."${packageName}"
+      ];
+      pydantic = (pkgs-final: pkgs-prev: {
+        pythonPackagesExtensions = pkgs-prev.pythonPackagesExtensions ++ [
           (final: prev: {
-            "${packageName}" = self.packages."${system}".default;
+            pydantic = pkgs-prev.python3Packages.callPackage ./pypi.nix {
+	      pname = "pydantic";
+	      version = "1.9.1";
+	      sha256 = "sha256-HtmHw/8p//f9jD6jo+qHetMQquLvmImhGeItPy2waRo=";
+	    };
+          })
+        ];
+      });
+      "${packageName}" = (pkgs-final: pkgs-prev: {
+        pythonPackagesExtensions = pkgs-prev.pythonPackagesExtensions ++ [
+          (final: prev: {
+            "${packageName}" = pkgs-prev.python3Packages.callPackage ./default.nix {
+              inherit packageName version;
+      	    };
           })
         ];
       });
-
-      dependencies = with pkgs; [ ];
     };
 
-    devShell."${system}" = let
-        pkgs = import nixpkgs {
-          inherit system;
-          overlays = [ self.packages."${system}".overlay ];
-        };
-
-        customPython = with pkgs; (pkgs.python3.withPackages (ps: [
-          ps."${packageName}"
-        ]));
-
-        packages = [ customPython ] ++ self.packages."${system}".dependencies;
-    in pkgs.mkShell {
-      inherit packages;
-    };
+    #devShell."${system}" = let
+    #  pkgs = import nixpkgs {
+    #    inherit system;
+    #    overlays = [ self.overlays.default ];
+    #  };
+    #  packages = [ (pkgs.python3.withPackages (ps: [ ps."${packageName}" ])) ];
+    #in pkgs.mkShell { inherit packages; };
 
   };
 }