Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/java/jar_genrule.bzl

@@ -0,0 +1,35 @@
+# 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.
+
+load("@prelude//:genrule.bzl", "process_genrule")
+load("@prelude//java:java_toolchain.bzl", "JavaToolchainInfo")
+load("@prelude//utils:utils.bzl", "expect")
+
+def jar_genrule_impl(ctx: "context") -> ["provider"]:
+    output_name = "{}.jar".format(ctx.label.name)
+    providers = process_genrule(ctx, output_name, None)
+    expect(
+        len(providers) == 1,
+        "expected exactly one provider of type DefaultInfo from {} ({})"
+            .format(ctx.label.name, providers),
+    )
+
+    default_info = providers[0]  # DefaultInfo type
+    outputs = default_info.default_outputs
+    expect(
+        len(outputs) == 1,
+        "expected exactly one output from {} ({})"
+            .format(ctx.label.name, outputs),
+    )
+    output_jar = outputs[0]
+
+    java_toolchain = ctx.attrs._java_toolchain[JavaToolchainInfo]
+    java_cmd = cmd_args(java_toolchain.java[RunInfo])
+    java_cmd.add("-jar", output_jar)
+
+    providers.append(RunInfo(args = java_cmd))
+    return providers