lib/modules.nix: Make entire definition list strict in config check

This is a non-trivial refactor that slightly changes the semantics
of the internal definition lists.
Whereas previously only individual list items would trigger the exception,
now the error is promoted to the whole list.
This is mostly ok, because we compute the value, it is wrong to ignore a definition.
However, we don't always compute the value. For instance `readOnly`
only needs to count definitions. That won't be possible anymore when
the error is raised for one of the items. As a consequence, an error
will be raised for the errant definition instead of the number of
definitions.

lib/modules.nix

@@ -556,48 +556,45 @@ let
                   )
                   subtree
               ) options);
+
+      # The root of any module definition must be an attrset.
+      checkedConfigs =
+        assert
+          lib.all
+            (c:
+              isAttrs c.config || throw ''
+                You're trying to define a value of type `${builtins.typeOf c.config}'
+                rather than an attribute set for the option
+                `${builtins.concatStringsSep "." prefix}'!
+
+                This usually happens if `${builtins.concatStringsSep "." prefix}' has option
+                definitions inside that are not matched. Please check how to properly define
+                this option by e.g. referring to `man 5 configuration.nix'!
+              ''
+            )
+            configs;
+        configs;
+
       # an attrset 'name' => list of submodules that define ‘name’.
       pushedDownDefinitionsByName =
         zipAttrsWith (n: concatLists)
           (map (module: let subtree = module.config; in
-              if !(builtins.isAttrs subtree) then
-                throw ''
-                  You're trying to define a value of type `${builtins.typeOf subtree}'
-                  rather than an attribute set for the option
-                  `${builtins.concatStringsSep "." prefix}'!
-
-                  This usually happens if `${builtins.concatStringsSep "." prefix}' has option
-                  definitions inside that are not matched. Please check how to properly define
-                  this option by e.g. referring to `man 5 configuration.nix'!
-                ''
-              else
                 mapAttrs
                   (n: value:
                     map (config: { inherit (module) file; inherit config; }) (pushDownProperties value)
                   )
                   subtree
-              ) configs);
+              ) checkedConfigs);
       # extract the definitions for each loc
       rawDefinitionsByName =
         zipAttrsWith (n: concatLists)
           (map (module: let subtree = module.config; in
-              if !(builtins.isAttrs subtree) then
-                throw ''
-                  You're trying to define a value of type `${builtins.typeOf subtree}'
-                  rather than an attribute set for the option
-                  `${builtins.concatStringsSep "." prefix}'!
-
-                  This usually happens if `${builtins.concatStringsSep "." prefix}' has option
-                  definitions inside that are not matched. Please check how to properly define
-                  this option by e.g. referring to `man 5 configuration.nix'!
-                ''
-              else
                 mapAttrs
                   (n: value:
                     [{ inherit (module) file; inherit value; }]
                   )
                 subtree
-              ) configs);
+              ) checkedConfigs);
 
       # Convert an option tree decl to a submodule option decl
       optionTreeToOption = decl: