Use overlay

flake.nix

@@ -15,22 +15,26 @@
   };
 
   outputs = inputs@{ self, flake-utils, nixpkgs, ... }:
-    flake-utils.lib.eachDefaultSystem (system:
+    let
+      pkgDrvDir = ./packages;
+      pkgDrvs = pkgs: pkgs.lib.attrsets.mapAttrs'
+        (pkgDrv: _: rec {
+          name = builtins.substring 0
+            (
+              builtins.sub (builtins.stringLength pkgDrv) 4
+            )
+            pkgDrv;
+          value = import (pkgDrvDir + "/${pkgDrv}") { inherit inputs name pkgs; };
+        })
+        (builtins.readDir pkgDrvDir);
+    in
+    {
+      overlay = _: prev: pkgDrvs prev;
+    } // flake-utils.lib.eachDefaultSystem (system:
       let
         pkgs = nixpkgs.legacyPackages.${system};
-        pkgDefDir = ./packages;
-        pkgDefs = pkgs.lib.attrsets.mapAttrs'
-          (pkgDef: _: {
-            name = builtins.substring 0
-              (
-                builtins.sub (builtins.stringLength pkgDef) 4
-              )
-              pkgDef;
-            value = import (pkgDefDir + "/${pkgDef}") { inherit inputs pkgs; };
-          })
-          (builtins.readDir pkgDefDir);
       in
       {
-        packages = pkgDefs;
+        packages = pkgDrvs pkgs;
       });
 }