modules/generic/meta-maintainers.nix: Simplify

There was no need for it to be this complicated.

Notably the maintainer check was never used anyways, because only its
merge function was used, which doesn't do a check

There is a minor functional change with this commit, which is that even
if explicitly `meta.maintainers = []`, that module will be in the result
when it wasn't before. I deem this insignificant.

modules/generic/meta-maintainers.nix

@@ -4,39 +4,12 @@
 let
   inherit (lib)
     mkOption
-    mkOptionType
     types
     ;
 
-  maintainer = mkOptionType {
-    name = "maintainer";
-    check = email: lib.elem email (lib.attrValues lib.maintainers);
-    merge = loc: defs: {
-      # lib.last: Perhaps this could be merged instead, if "at most once per module"
-      # is a problem (see option description).
-      ${(lib.last defs).file} = (lib.last defs).value;
-    };
-  };
-
-  listOfMaintainers = types.listOf maintainer // {
-    merge =
-      loc: defs:
-      lib.zipAttrs (
-        lib.flatten (
-          lib.imap1 (
-            n: def:
-            lib.imap1 (
-              m: def':
-              maintainer.merge (loc ++ [ "[${toString n}-${toString m}]" ]) [
-                {
-                  inherit (def) file;
-                  value = def';
-                }
-              ]
-            ) def.value
-          ) defs
-        )
-      );
+  # The resulting value of this type shows where all values were defined
+  sourceList = types.listOf types.raw // {
+    merge = loc: defs: lib.listToAttrs (lib.map ({ file, value }: lib.nameValuePair file value) defs);
   };
 in
 {
@@ -44,7 +17,7 @@ in
   options = {
     meta = {
       maintainers = mkOption {
-        type = listOfMaintainers;
+        type = sourceList;
         default = [ ];
         example = lib.literalExpression "[ lib.maintainers.alice lib.maintainers.bob ]";
         description = ''