Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/go/toolchain.bzl

@@ -0,0 +1,43 @@
+# 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.
+
+GoToolchainInfo = provider(fields = [
+    "assembler",
+    "cgo",
+    "cgo_wrapper",
+    "compile_wrapper",
+    "compiler",
+    "cover",
+    "cover_srcs",
+    "cxx_toolchain_for_linking",
+    "env_go_arch",
+    "env_go_os",
+    "env_go_root",
+    "external_linker_flags",
+    "filter_srcs",
+    "go",
+    "linker",
+    "packer",
+    "tags",
+])
+
+def get_toolchain_cmd_args(toolchain: "GoToolchainInfo", go_root = True) -> "cmd_args":
+    cmd = cmd_args("env")
+    if toolchain.env_go_arch != None:
+        cmd.add("GOARCH={}".format(toolchain.env_go_arch))
+    if toolchain.env_go_os != None:
+        cmd.add("GOOS={}".format(toolchain.env_go_os))
+    if go_root and toolchain.env_go_root != None:
+        cmd.add(cmd_args(toolchain.env_go_root, format = "GOROOT={}"))
+
+    # CGO is enabled by default for native compilation, but we need to set it
+    # explicitly for cross-builds:
+    # https://go-review.googlesource.com/c/go/+/12603/2/src/cmd/cgo/doc.go
+    if toolchain.cgo != None:
+        cmd.add("CGO_ENABLED=1")
+
+    return cmd