Add the tool "nixos-typecheck" that can check an option declaration to:

- Enforce that an option declaration has a "defaultText" if and only if the
   type of the option derives from "package", "packageSet" or "nixpkgsConfig"
   and if a "default" attribute is defined.

 - Enforce that the value of the "example" attribute is wrapped with "literalExample"
   if the type of the option derives from "package", "packageSet" or "nixpkgsConfig".

 - Warn if a "defaultText" is defined in an option declaration if the type of
   the option does not derive from "package", "packageSet" or "nixpkgsConfig".

 - Warn if no "type" is defined in an option declaration.

nixos/modules/services/web-servers/apache-httpd/default.nix

@@ -48,8 +48,7 @@ let
           if svc ? function then svc.function
           else import (toString "${toString ./.}/${if svc ? serviceType then svc.serviceType else svc.serviceName}.nix");
         config = (evalModules
-          { modules = [ { options = res.options; config = svc.config or svc; } ];
-            check = false;
+          { modules = [ { options = res.options; config = svc.config or svc; } ] ++ [ { _module.check = false; } ];
           }).config;
         defaults = {
           extraConfig = "";