Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/java/dex_toolchain.bzl

@@ -0,0 +1,16 @@
+# 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.
+
+# TODO(T107163344) These should be part of the Android toolchain!
+# Move out once we have overlays.
+DexToolchainInfo = provider(
+    "Dex toolchain info",
+    fields = [
+        "android_jar",
+        "d8_command",
+    ],
+)