Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/android/exopackage.bzl

@@ -0,0 +1,27 @@
+# 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.
+
+SECONDARY_DEX = 1
+NATIVE_LIBRARY = 2
+RESOURCES = 4
+MODULES = 8
+ARCH64 = 16
+
+def get_exopackage_flags(exopackage_modes: [str.type]) -> int.type:
+    flags = 0
+
+    for (name, flag) in [
+        ("secondary_dex", SECONDARY_DEX),
+        ("native_library", NATIVE_LIBRARY),
+        ("resources", RESOURCES),
+        ("modules", MODULES),
+        ("arch64", ARCH64),
+    ]:
+        if name in exopackage_modes:
+            flags += flag
+
+    return flags