refactor(flake): inline package overlay

src/flake/default.nix

@@ -6,7 +6,12 @@
   system,
 }:
 let
-  overlay = import ./overlay.nix { inherit root; };
+  overlay = final: _prev: {
+    alexion = import ./packages {
+      pkgs = final;
+      inherit root;
+    };
+  };
   pkgs = import inputs.nixpkgs {
     inherit system;
     overlays = [ overlay ];

src/flake/overlay.nix

@@ -1,7 +0,0 @@
-{ root }:
-final: _prev: {
-  alexion = import ./packages {
-    pkgs = final;
-    inherit root;
-  };
-}