chore: refactor flake layout

Signed-off-by: Lander Van den Bulcke <landervandenbulcke@gmail.com>

overlays/default.nix

@@ -0,0 +1,20 @@
+{ inputs, ... }:
+{
+  # Import custom packages
+  additions = final: _prev: import ../pkgs final.pkgs;
+
+  # Add modifications here:
+  modifications = final: prev: {
+    # example = prev.example.overrideAttrs (oldAttrs: rec {
+    # ...
+    # });
+  };
+
+  # add nixpkgs unstable (accessible through pkgs.unstable)
+  unstable-packages = final: _prev: {
+    unstable = import inputs.nixpkgs-unstable {
+      system = final.system;
+      config.allowUnfree = true;
+    };
+  };
+}