Vendor dependencies

Let's see how I like this workflow.

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

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+# 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.
+
+AbiGenerationMode = enum("class", "source", "source_only")
+
+JavacProtocol = enum("classic", "javacd")
+
+JavaPlatformInfo = provider(
+    "Java platform info",
+    fields = [
+        "name",
+    ],
+)
+
+JavaToolchainInfo = provider(
+    "Java toolchain info",
+    fields = [
+        "abi_generation_mode",
+        "ast_dumper",
+        "bootclasspath_7",
+        "bootclasspath_8",
+        "class_abi_generator",
+        "compile_and_package",
+        "dep_files",
+        "fallback_javac",
+        "fat_jar",
+        "fat_jar_main_class_lib",
+        "jar",
+        "jar_builder",
+        "java",
+        "java_for_tests",
+        "javac",
+        "javac_protocol",
+        "src_dir_helper",
+        "source_level",
+        "src_root_elements",
+        "src_root_prefixes",
+        "target_level",
+        "used_classes_to_dep_file",
+        "zip_scrubber",
+        "is_bootstrap_toolchain",
+    ],
+)
+
+JavaTestToolchainInfo = provider(
+    "Java test toolchain info",
+    fields = [
+        "java_custom_class_loader_class",
+        "java_custom_class_loader_library_jar",
+        "java_custom_class_loader_vm_args",
+        "test_runner_library_jar",
+        "junit_test_runner_main_class_args",
+        "junit5_test_runner_main_class_args",
+        "testng_test_runner_main_class_args",
+        "list_class_names",
+        "use_java_custom_class_loader",
+    ],
+)
+
+# prebuilt_jar needs so little of the Java toolchain that it's worth
+# giving it its own to reduce the occurrence of cycles as we add
+# more Java- and Kotlin-built tools to the Java and Kotlin toolchains
+PrebuiltJarToolchainInfo = provider(
+    "prebuilt_jar toolchain info",
+    fields = [
+        "class_abi_generator",
+        "is_bootstrap_toolchain",
+    ],
+)