refactor: Tidy flake.nix and break it apart

modules/nixos/defaults.nix

@@ -0,0 +1,15 @@
+{ config, lib, pkgs, ... }:
+
+let
+  # TODO: Move to a fucking utility library already!
+  listNixFilesInDirectory = dir:
+    builtins.attrNames
+      (lib.filterAttrs
+        (k: _v: lib.hasSuffix ".nix" k)
+        (builtins.readDir dir));
+in {
+  imports =
+    builtins.map
+      (k: ./defaults/${k})
+      (listNixFilesInDirectory ./defaults);
+}