Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/kotlin/kotlin_toolchain.bzl

@@ -0,0 +1,25 @@
+# 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.
+
+KotlincProtocol = enum("classic", "kotlincd")
+
+KotlinToolchainInfo = provider(
+    "Kotlin toolchain info",
+    fields = [
+        "annotation_processing_jar",
+        "compile_kotlin",
+        "kapt_base64_encoder",
+        "kotlinc",
+        "kotlinc_classpath",
+        "kotlinc_protocol",
+        "kotlin_stdlib",
+        "kotlin_home_libraries",
+        "kosabi_stubs_gen_plugin",
+        "kosabi_applicability_plugin",
+        "kosabi_jvm_abi_gen_plugin",
+    ],
+)