Reorganize configuration root

While I might still bikeshed how the rest of the configuration is
sourced, at least the import points should be stable.

hm/default.nix

@@ -0,0 +1,12 @@
+{ ... }:
+{
+  imports = [
+    ../options.nix
+    ./common.nix
+    ./desktop.nix
+    ./dev.nix
+    ./extra.nix
+    ./style.nix
+    ./vim.nix
+  ];
+}