Vendor dependencies

Let's see how I like this workflow.

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

@@ -0,0 +1,30 @@
+# 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//utils:utils.bzl", "value_or")
+
+def go_attr_pkg_name(ctx: "context") -> str.type:
+    """
+    Return the Go package name for the given context corresponing to a rule.
+    """
+    return value_or(ctx.attrs.package_name, ctx.label.package)
+
+def merge_pkgs(pkgss: [{str.type: "artifact"}]) -> {str.type: "artifact"}:
+    """
+    Merge mappings of packages into a single mapping, throwing an error on
+    conflicts.
+    """
+
+    all_pkgs = {}
+
+    for pkgs in pkgss:
+        for name, path in pkgs.items():
+            if name in pkgs and pkgs[name] != path:
+                fail("conflict for package {!r}: {} and {}".format(name, path, all_pkgs[name]))
+            all_pkgs[name] = path
+
+    return all_pkgs