ci,modules: Backport additions of #431450

Reason: keep ci directory in sync
- https://github.com/NixOS/nixpkgs/pull/431450#issuecomment-3209546418

This requires that we have a modules directory, in which case the
easy and robust solution is to only port the addition parts of the refactor.
It's about as easy as a .keep file, but more useful.

This means that some duplication is created, but we avoid backporting the
changes to the documentation generation, which is a somewhat complex
component I'd rather not touch until these changes have been proven out
on unstable.

ci/eval/default.nix

@@ -30,6 +30,7 @@ let
           "doc"
           "lib"
           "maintainers"
+          "modules"
           "nixos"
           "pkgs"
           ".version"

modules/README.md

@@ -0,0 +1,9 @@
+# `<nixpkgs>/modules`
+
+This directory hosts subdirectories representing each module [class](https://nixos.org/manual/nixpkgs/stable/#module-system-lib-evalModules-param-class) for which the `nixpkgs` repository has user-importable modules.
+
+Exceptions:
+- `_class = "nixos";` modules go in the `<nixpkgs>/nixos/modules` tree
+- modules whose only purpose is to test code in this repository
+
+The emphasis is on _importable_ modules, i.e. ones that aren't inherent to and built into the Module System application.

modules/generic/meta-maintainers.nix

@@ -0,0 +1,63 @@
+# Test:
+#   ./meta-maintainers/test.nix
+{ lib, ... }:
+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
+        )
+      );
+  };
+in
+{
+  _class = null; # not specific to NixOS
+  options = {
+    meta = {
+      maintainers = mkOption {
+        type = listOfMaintainers;
+        default = [ ];
+        example = lib.literalExpression ''[ lib.maintainers.alice lib.maintainers.bob ]'';
+        description = ''
+          List of maintainers of each module.
+          This option should be defined at most once per module.
+
+          The option value is not a list of maintainers, but an attribute set that maps module file names to lists of maintainers.
+        '';
+      };
+    };
+  };
+  meta.maintainers = with lib.maintainers; [
+    pierron
+    roberth
+  ];
+}

modules/generic/meta-maintainers/test.nix

@@ -0,0 +1,34 @@
+# Run:
+#   $ nix-instantiate --eval 'modules/generic/meta-maintainers/test.nix'
+#
+# Expected output:
+#   { }
+#
+# Debugging:
+#   drop .test from the end of this file, then use nix repl on it
+rec {
+  lib = import ../../../lib;
+
+  example = lib.evalModules {
+    modules = [
+      ../meta-maintainers.nix
+      {
+        _file = "eelco.nix";
+        meta.maintainers = [ lib.maintainers.eelco ];
+      }
+    ];
+  };
+
+  test =
+    assert
+      example.config.meta.maintainers == {
+        ${toString ../meta-maintainers.nix} = [
+          lib.maintainers.pierron
+          lib.maintainers.roberth
+        ];
+        "eelco.nix" = [ lib.maintainers.eelco ];
+      };
+    { };
+
+}
+.test