Vendor dependencies

Let's see how I like this workflow.

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

@@ -0,0 +1,23 @@
+# 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.
+
+def filegroup_impl(ctx):
+    """
+    Creates a directory that contains links to the list of srcs
+
+    The output is a directory that uses `name` for its name, and each symlink
+    is based on the `short_path` for the provided `src`.
+    """
+
+    if type(ctx.attrs.srcs) == type({}):
+        srcs = ctx.attrs.srcs
+    else:
+        srcs = {src.short_path: src for src in ctx.attrs.srcs}
+
+    # It seems that buck1 always copies, and that's important for Python rules
+    output = ctx.actions.copied_dir(ctx.label.name, srcs)
+    return [DefaultInfo(default_outputs = [output])]