Migrating to executorch (#2768)

Summary:
title.

Pull Request resolved: #2768

Test Plan:
```
[[email protected] /data/sandcastle/boxes/fbsource (82a3fe9ca)]$ buck2 test //xplat/cria/core:cria
Watchman fresh instance: new mergebase, cleared graph state, cleared dep files
Soft Error: source_file_missing: Source file `Launder.h` does not exist as a member of package `fbsource//xplat/folly/lang`.
Soft Error: source_file_missing: Source file `Launder.h` does not exist as a member of package `fbsource//xplat/folly/lang`.
Soft Error: source_file_missing: Source file `Launder.h` does not exist as a member of package `fbsource//xplat/folly/lang`.
Soft Error: source_file_missing: Source file `Launder.h` does not exist as a member of package `fbsource//xplat/folly/lang`.
Soft Error: source_file_missing: Source file `Launder.h` does not exist as a member of package `fbsource//xplat/folly/lang`.
Soft Error: source_file_missing: Source file `Launder.h` does not exist as a member of package `fbsource//xplat/folly/lang`.
Soft Error: source_file_missing: Source file `Launder.h` does not exist as a member of package `fbsource//xplat/folly/lang`.
Buck UI: https://www.internalfb.com/buck2/24c6e2bb-c070-4a31-a12e-e3b4821df7dd
Test UI: https://www.internalfb.com/intern/testinfra/testrun/10133099189773521
Network: Up: 763KiB  Down: 508MiB  (reSessionID-13d45184-0acc-48ee-96e7-1979b53d74a7)
Jobs completed: 4460. Time elapsed: 41.9s.
Cache hits: 96%. Commands: 1015 (cached: 973, remote: 0, local: 42)
Tests finished: Pass 9. Fail 0. Fatal 0. Skip 0. Build failure 0
```

Reviewed By: mergennachin

Differential Revision: D55505037

Pulled By: ChristianWLang

fbshipit-source-id: 09c980f80aa7839ca41edf97f7bbd724503950de

Author: ChristianWLang

extension/parallel/targets.bzl

@@ -20,6 +20,7 @@ def define_common_targets():
             ],
             visibility = [
                 "//executorch/...",
+                "@EXECUTORCH_CLIENTS",
             ],
             deps = [
                 "//executorch/backends/xnnpack/threadpool:threadpool",