Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/alias.bzl

@@ -0,0 +1,22 @@
+# 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.
+
+# Implementation of aliases build rules.
+
+def alias_impl(ctx: "context") -> ["provider"]:
+    return ctx.attrs.actual.providers
+
+def configured_alias_impl(ctx: "context") -> ["provider"]:
+    if ctx.attrs.configured_actual != None:
+        return ctx.attrs.configured_actual.providers
+    if ctx.attrs.fallback_actual != None:
+        return ctx.attrs.fallback_actual.providers
+    fail("must set one of `configured_actual` or `fallback_actual`")
+
+def versioned_alias_impl(_ctx: "context") -> ["provider"]:
+    # Should be intercepted in macro stub and converted to `alias`.
+    fail("unsupported")