Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/js/js_providers.bzl

@@ -0,0 +1,47 @@
+# 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 _artifacts(value: "artifact"):
+    return value
+
+TransitiveOutputsTSet = transitive_set(args_projections = {"artifacts": _artifacts})
+
+JsLibraryInfo = provider(
+    fields = [
+        "output",  # "artifact"
+        "transitive_outputs",  # ["TransitiveOutputsTSet", None]
+    ],
+)
+
+JsBundleInfo = provider(
+    fields = [
+        "bundle_name",  # str.type
+        # Directory containing the built JavaScript.
+        "built_js",  # "artifact",
+        # Source map belonging to the built JavaScript.
+        "source_map",  # "artifact",
+        # Directory containing the resources (or assets) used by the bundled JavaScript source code.
+        "res",  # ["artifact", None]
+        # Directory containing various metadata that can be used by dependent rules but are not
+        # meant to be shipped with the application.
+        "misc",  # "artifact"
+        # Dependencies graph file associated with the built JavaScript.
+        "dependencies_file",  # "artifact"
+    ],
+)
+
+def get_transitive_outputs(
+        actions: "actions",
+        value: ["artifact", None] = None,
+        deps: [JsLibraryInfo.type] = []) -> TransitiveOutputsTSet.type:
+    kwargs = {}
+    if value:
+        kwargs["value"] = value
+    if deps:
+        kwargs["children"] = filter(None, [js_library_info.transitive_outputs for js_library_info in deps])
+
+    return actions.tset(TransitiveOutputsTSet, **kwargs)