Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/utils/dicts.bzl

@@ -0,0 +1,27 @@
+# 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.
+
+load(
+    "@prelude//utils:utils.bzl",
+    "expect",
+)
+
+_DEFAULT_FMT = "found different values for key \"{0}\": {} != {}"
+
+def update_x(dst: {"_a": "_b"}, k: "_a", v: "_b", fmt = _DEFAULT_FMT):
+    p = dst.setdefault(k, v)
+    expect(p == v, fmt, k, p, v)
+
+def merge_x(dst: {"_a": "_b"}, src: {"_a": "_b"}, fmt = _DEFAULT_FMT):
+    for k, v in src.items():
+        update_x(dst, k, v, fmt = fmt)
+
+def flatten_x(ds: [{"_a": "_b"}], fmt = _DEFAULT_FMT) -> {"_a": "_b"}:
+    out = {}
+    for d in ds:
+        merge_x(out, d, fmt = fmt)
+    return out