modules/misc: include assertions and meta modules in the repo

Based on the modules we previously imported from NixOS.

This will allow us to drop the `defaultPkgs` specialArg and avoids
needing the proposed `pkgsPath` specialArg.

modules/misc/meta.nix

@@ -0,0 +1,56 @@
+{ lib, ... }:
+# Based on https://github.com/NixOS/nixpkgs/blob/814a4e48/nixos/modules/misc/meta.nix
+let
+  maintainer = lib.mkOptionType {
+    name = "maintainer";
+    check = m: lib.elem m (lib.attrValues lib.maintainers);
+    merge =
+      loc: defs:
+      let
+        def = lib.last defs;
+      in
+      {
+        ${def.file} = def.value;
+      };
+  };
+
+  listOfMaintainers = lib.types.listOf maintainer // {
+    # Returns attrset of
+    #   { "module-file" = [
+    #        "maintainer1 <first@nixos.org>"
+    #        "maintainer2 <second@nixos.org>" ];
+    #   }
+    merge =
+      loc: defs:
+      lib.pipe defs [
+        (lib.imap1 (
+          n: def:
+          lib.imap1 (
+            m: value:
+            maintainer.merge (loc ++ [ "[${toString n}-${toString m}]" ]) [
+              {
+                inherit (def) file;
+                inherit value;
+              }
+            ]
+          ) def.value
+        ))
+        lib.flatten
+        lib.zipAttrs
+      ];
+  };
+in
+{
+  options.meta = {
+    maintainers = lib.mkOption {
+      type = listOfMaintainers;
+      internal = true;
+      default = [ ];
+      example = lib.literalExpression "[ lib.maintainers.all ]";
+      description = ''
+        List of maintainers of each module.  This option should be defined at
+        most once per module.
+      '';
+    };
+  };
+}