Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/apple/apple_sdk.bzl

@@ -0,0 +1,15 @@
+# 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//apple:apple_toolchain_types.bzl", "AppleToolchainInfo")
+
+def get_apple_sdk_name(ctx: "context") -> str.type:
+    """
+    Get the SDK defined on the toolchain.
+    Will throw if the `_apple_toolchain` is not present.
+    """
+    return ctx.attrs._apple_toolchain[AppleToolchainInfo].sdk_name