Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/toolchains/BUCK

@@ -0,0 +1,21 @@
+load("@prelude//toolchains:cxx.bzl", "system_cxx_toolchain")
+load("@prelude//toolchains:python.bzl", "system_python_bootstrap_toolchain")
+load("@prelude//toolchains:rust.bzl", "system_rust_toolchain")
+
+system_cxx_toolchain(
+    name = "cxx",
+    visibility = ["PUBLIC"],
+)
+
+system_python_bootstrap_toolchain(
+    name = "python_bootstrap",
+    visibility = ["PUBLIC"],
+)
+
+system_rust_toolchain(
+    name = "rust",
+    default_edition = None,
+    rustc_flags = ["-Clink-arg=-fuse-ld=lld"],
+    rustdoc_flags = ["-Zunstable-options"],  # doc builds use unstable '--extern-html-root-url'
+    visibility = ["PUBLIC"],
+)