porthos: split into modules

I have separated the modules into host-specific settings, and generic
settings that ought to be shared by every host.

I only have the 'porthos' host for now, but intend to also add my laptop
'aramis' at some point to this repository.

modules/default.nix

@@ -0,0 +1,11 @@
+# Common modules
+{ ... }:
+
+{
+  imports = [
+    ./language.nix
+    ./nix.nix
+    ./packages.nix
+    ./users.nix
+  ];
+}