Started migration towards flake-parts

Plus shit ton many changes I still had to commit

packages/default.nix

@@ -0,0 +1,37 @@
+{
+  inputs,
+  lib,
+  config,
+  ...
+}: {
+  options.fleet.overlays = let
+    overlayType = lib.mkOptionType {
+      name = "nixpkgs-overlay";
+      description = "nixpkgs overlay";
+      check = lib.isFunction;
+      merge = lib.mergeOneOption;
+    };
+  in
+    lib.mkOption {
+      description = "Nixpkgs overlays to apply at flake level (not in hosts)";
+      type = lib.types.listOf overlayType;
+      default = with inputs; [
+        agenix.overlays.default
+        comma.overlays.default
+        nur.overlay
+        nil.overlays.default
+      ];
+    };
+
+  config.perSystem = {
+    system,
+    lib,
+    ...
+  }: {
+    _module.args.pkgs =
+      lib.foldl
+      (legacyPackages: overlay: legacyPackages.extend overlay)
+      inputs.nixpkgsUnstable.legacyPackages.${system}
+      config.fleet.overlays;
+  };
+}