build-clan: move clan attribute relates modules into subfolder

lib/modules/clan/secrets.nix

@@ -0,0 +1,18 @@
+{
+  lib,
+  ...
+}:
+let
+  inherit (lib) types;
+in
+{
+  options = {
+    age.plugins = lib.mkOption {
+      type = types.listOf (types.strMatching "age-plugin-.*");
+      default = [ ];
+      description = ''
+        A list of age plugins which must be available in the shell when encrypting and decrypting secrets.
+      '';
+    };
+  };
+}