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/services/unison.nix

@@ -93,6 +93,11 @@ in {
   };
 
   config = mkIf cfg.enable {
+    assertions = [
+      (lib.hm.assertions.assertPlatform "services.unison" pkgs
+        lib.platforms.linux)
+    ];
+
     systemd.user.services = makeDefs (name: pairCfg: {
       Unit = {
         Description = "Unison pair sync (${name})";