Move platform check into modules

Before, loading a module would be guarded by an optional platform
condition. This made it possible to avoid loading and evaluating a
module if it did not support the host platform.

Unfortunately, this made it impossible to share a single configuration
between GNU/Linux and Darwin hosts, which some wish to do.

This removes the conditional load and instead inserts host platform
assertions in the modules that are platform specific.

Fixes #1906

modules/config/i18n.nix

@@ -36,7 +36,7 @@ let
 in {
   meta.maintainers = with maintainers; [ midchildan ];
 
-  config = {
+  config = mkIf pkgs.hostPlatform.isLinux {
     # For shell sessions.
     home.sessionVariables = localeVars;