refactor(flake): move flake modules into separate subdirectories

Also put `outputs` before `inputs` in `flake.nix` (readability)

modules/flake/pkgs/default.nix

@@ -0,0 +1,23 @@
+{ inputs, self, lib, config, ... }:
+
+{
+  perSystem = { system, ... }: {
+    _module.args.pkgs = import inputs.nixpkgs {
+      inherit system;
+      # WARN: not including `self.packages` overylay
+      #       because it causes an infinite recursion
+      overlays = lib.attrValues self.overlays ++ [
+        inputs.neovim-nightly-overlay.overlays.default
+        inputs.zig-overlay.overlays.default
+        inputs.nix-topology.overlays.default
+        inputs.wired.overlays.default
+        # nix-on-droid overlay (needed for `proot`)
+        inputs.nix-on-droid.overlays.default
+      ];
+      config = {
+        # TODO: per machine?
+        allowUnfree = true;
+      };
+    };
+  };
+}