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

lib/tests/modules/assertions/condition-true.nix

@@ -0,0 +1,8 @@
+{
+
+  _module.checks.test = {
+    check = true;
+    message = "Assertion failed";
+  };
+
+}

lib/tests/modules/assertions/enable-false.nix

@@ -2,6 +2,7 @@
 
   _module.checks.test = {
     enable = false;
+    check = false;
     message = "Assertion failed";
   };
 

lib/tests/modules/assertions/multi.nix

@@ -2,20 +2,20 @@
 
   _module.checks = {
     test1 = {
-      enable = true;
+      check = false;
       message = "Assertion 1 failed";
     };
     test2 = {
-      enable = true;
+      check = false;
       message = "Assertion 2 failed";
     };
     test3 = {
-      enable = true;
+      check = false;
       message = "Warning 3 failed";
       type = "warning";
     };
     test4 = {
-      enable = true;
+      check = false;
       message = "Warning 4 failed";
       type = "warning";
     };

lib/tests/modules/assertions/simple.nix

@@ -1,6 +1,6 @@
 {
   _module.checks.test = {
-    enable = true;
+    check = false;
     message = "Assertion failed";
   };
 }

lib/tests/modules/assertions/submodule-attrsOf-attrsOf.nix

@@ -4,7 +4,7 @@
     default = { bar.baz = {}; };
     type = lib.types.attrsOf (lib.types.attrsOf (lib.types.submodule {
       _module.checks.test = {
-        enable = true;
+        check = false;
         message = "Assertion failed";
       };
     }));

lib/tests/modules/assertions/submodule-attrsOf.nix

@@ -4,7 +4,7 @@
     default = { bar = {}; };
     type = lib.types.attrsOf (lib.types.submodule {
       _module.checks.test = {
-        enable = true;
+        check = false;
         message = "Assertion failed";
       };
     });

lib/tests/modules/assertions/submodule.nix

@@ -4,7 +4,7 @@
     default = {};
     type = lib.types.submodule {
       _module.checks.test = {
-        enable = true;
+        check = false;
         message = "Assertion failed";
       };
     };

lib/tests/modules/assertions/underscore-attributes.nix

@@ -1,7 +1,7 @@
 {
 
   _module.checks._test = {
-    enable = true;
+    check = false;
     message = "Assertion failed";
   };
 

lib/tests/modules/assertions/warning.nix

@@ -1,7 +1,7 @@
 {
 
   _module.checks.test = {
-    enable = true;
+    check = false;
     type = "warning";
     message = "Warning message";
   };