Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/configurations/util.bzl

@@ -0,0 +1,32 @@
+# 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 _configuration_info_union(infos):
+    if len(infos) == 0:
+        return ConfigurationInfo(
+            constraints = {},
+            values = {},
+        )
+    if len(infos) == 1:
+        return infos[0]
+    constraints = {k: v for info in infos for (k, v) in info.constraints.items()}
+    values = {k: v for info in infos for (k, v) in info.values.items()}
+    return ConfigurationInfo(
+        constraints = constraints,
+        values = values,
+    )
+
+def _constraint_values_to_configuration(values):
+    return ConfigurationInfo(constraints = {
+        info[ConstraintValueInfo].setting.label: info[ConstraintValueInfo]
+        for info in values
+    }, values = {})
+
+util = struct(
+    configuration_info_union = _configuration_info_union,
+    constraint_values_to_configuration = _constraint_values_to_configuration,
+)