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/packages.nix

@@ -0,0 +1,16 @@
+# Common packages
+{ pkgs, ... }:
+
+{
+  # List packages installed in system profile. To search, run:
+  # $ nix search wget
+  environment.systemPackages = with pkgs; [
+    git
+    git-crypt
+    mosh
+    vim
+    wget
+  ];
+
+  nixpkgs.config.allowUnfree = true; # Because I don't care *that* much.
+}