Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/apple/swift_toolchain.bzl

@@ -0,0 +1,65 @@
+# 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(":apple_sdk_module.bzl", "create_sdk_modules_graph")
+load(":swift_toolchain_types.bzl", "SdkUncompiledModuleInfo", "SwiftToolchainInfo")
+
+def swift_toolchain_impl(ctx):
+    compiler = cmd_args(ctx.attrs._swiftc_wrapper[RunInfo]).add(ctx.attrs.swiftc[RunInfo])
+    compiler_flags = ctx.attrs.swiftc_flags
+    sdk_path = ctx.attrs._internal_sdk_path or ctx.attrs.sdk_path
+    resource_dir = ctx.attrs.resource_dir
+
+    toolchain_context = struct(
+        compiler = compiler,
+        sdk_path = sdk_path,
+        compiler_flags = compiler_flags,
+        swift_resource_dir = resource_dir,
+    )
+
+    compiled_sdk_module_providers = {}
+
+    sdk_uncompiled_module_infos = filter(None, [d.get(SdkUncompiledModuleInfo) for d in ctx.attrs.sdk_modules])
+    for uncompiled_swift_module_info in sdk_uncompiled_module_infos:
+        create_sdk_modules_graph(
+            ctx,
+            compiled_sdk_module_providers,
+            uncompiled_swift_module_info,
+            toolchain_context,
+        )
+
+    compiled_sdk_swift_module_providers = {
+        info.module_name: info
+        for _, info in compiled_sdk_module_providers.items()
+        if info.is_swiftmodule
+    }
+
+    compiled_sdk_clang_module_providers = {
+        info.module_name: info
+        for _, info in compiled_sdk_module_providers.items()
+        if not info.is_swiftmodule
+    }
+
+    return [
+        DefaultInfo(),
+        SwiftToolchainInfo(
+            architecture = ctx.attrs.architecture,
+            can_toolchain_emit_obj_c_header_textually = ctx.attrs.can_toolchain_emit_obj_c_header_textually,
+            # TODO(T99038725): until we add -debug-compilation-dir we need to wrap
+            # the Swift invocations so that we can apply a debug prefix map for
+            # the current directory while maintaining cache hit.
+            compiled_sdk_clang_modules = compiled_sdk_clang_module_providers,
+            compiled_sdk_swift_modules = compiled_sdk_swift_module_providers,
+            compiler = compiler,
+            compiler_flags = compiler_flags,
+            prefix_serialized_debugging_options = ctx.attrs.prefix_serialized_debug_info,
+            resource_dir = resource_dir,
+            sdk_path = sdk_path,
+            swift_stdlib_tool = ctx.attrs.swift_stdlib_tool[RunInfo],
+            swift_stdlib_tool_flags = ctx.attrs.swift_stdlib_tool_flags,
+        ),
+    ]