lib/modules: Introduce _module.checks.*.check

Previously the .enable option was used to encode the condition as well,
which lead to some oddness:
- In order to encode an assertion, one had to invert it
- To disable a check, one had to mkForce it

By introducing a separate .check option this is solved because:
- It can be used to encode assertions
- Disabling is done separately with .enable option, whose default can be
  overridden without a mkForce

nixos/modules/misc/assertions.nix

@@ -36,7 +36,7 @@ with lib;
       name = "_${toString n}";
       isWarning = lib.isString value;
       result = {
-        enable = if isWarning then true else ! value.assertion;
+        check = if isWarning then false else value.assertion;
         type = if isWarning then "warning" else "error";
         message = if isWarning then value else value.message;
       };