Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/user/rule_spec.bzl

@@ -0,0 +1,13 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under both the MIT license found in the
+# LICENSE-MIT file in the root directory of this source tree and the Apache
+# License, Version 2.0 found in the LICENSE-APACHE file in the root directory
+# of this source tree.
+
+RuleRegistrationSpec = record(
+    name = field(str.type),
+    impl = field("function"),
+    attrs = field({str.type: "attribute"}),
+    cfg = field([None, "transition"], None),
+)