Vendor dependencies

Let's see how I like this workflow.

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

@@ -0,0 +1,21 @@
+# 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 the `export_file` build rule.
+
+def export_file_impl(ctx: "context") -> [DefaultInfo.type]:
+    # mode is "copy" or "reference", defaulting to copy
+    copy = ctx.attrs.mode != "reference"
+
+    if copy:
+        dest = ctx.label.name if ctx.attrs.out == None else ctx.attrs.out
+        output = ctx.actions.copy_file(dest, ctx.attrs.src)
+    elif ctx.attrs.out != None:
+        fail("export_file does not allow specifying `out` without also specifying `mode = 'copy'`")
+    else:
+        output = ctx.attrs.src
+    return [DefaultInfo(default_outputs = [output])]