modules/files: don't include modules in the docs

This means we no longer need to spoof the module in the docs implementation.

Instead, we supply the (optional) special arg `isDocs` to `evalModules`.

docs/mdbook/default.nix

@@ -14,6 +14,7 @@ let
     specialArgs = {
       inherit helpers;
       defaultPkgs = pkgs;
+      isDocs = true;
     };
   };