buildClan function: export all machines via nixosModules/darwinModules

We want each machine not only to be exposed via nixosConfigurations but also as a module.
This allows re-importing the machine in tests and override the architecture for example.

docs/site/guides/getting-started/secrets.md

@@ -100,7 +100,7 @@ are loaded when using Clan:
       };
     in
     {
-      inherit (clan) nixosConfigurations clanInternals;
+      inherit (clan) nixosConfigurations nixosModules clanInternals;
 
       # elided for brevity
     };