build_variables.bzl: make THREADPOOL_SRCS usable in BUCK (pytorch#13354)

Making the structure in this file mirror buck's worldview more closely
when it makes sense. pytorch#8268

Differential Revision:
[D80187442](https://our.internmc.facebook.com/intern/diff/D80187442)

Author: swolchok

extension/threadpool/targets.bzl

@@ -1,4 +1,5 @@
 load("@fbsource//xplat/executorch/backends/xnnpack/third-party:third_party_libs.bzl", "third_party_dep")
+load("@fbsource//xplat/executorch/build:build_variables.bzl", "THREADPOOL_SRCS")
 load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "runtime")
 
 def define_common_targets():
@@ -8,11 +9,9 @@ def define_common_targets():
     TARGETS and BUCK files that call this function.
     """
 
-    _THREADPOOL_SRCS = [
-        "thread_parallel.cpp",
-        "threadpool.cpp",
-        "threadpool_guard.cpp",
-    ] + (["fb/threadpool_use_n_threads.cpp"] if not runtime.is_oss else [])
+    _THREADPOOL_SRCS = THREADPOOL_SRCS + (
+        ["fb/threadpool_use_n_threads.cpp"] if not runtime.is_oss else []
+    )
 
     _THREADPOOL_HEADERS = [
         "threadpool.h",

shim_et/xplat/executorch/build/build_variables.bzl

@@ -363,12 +363,14 @@ EXTENSION_TENSOR_SRCS = [
     "extension/tensor/tensor_ptr_maker.cpp",
 ]
 
-EXTENSION_THREADPOOL_SRCS = [
-    "extension/threadpool/thread_parallel.cpp",
-    "extension/threadpool/threadpool.cpp",
-    "extension/threadpool/threadpool_guard.cpp",
+THREADPOOL_SRCS = [
+    "thread_parallel.cpp",
+    "threadpool.cpp",
+    "threadpool_guard.cpp",
 ]
 
+EXTENSION_THREADPOOL_SRCS = ["extension/threadpool/" + x for x in THREADPOOL_SRCS]
+
 EXTENSION_TRAINING_SRCS = [
     "extension/data_loader/file_data_loader.cpp",
     "extension/data_loader/mmap_data_loader.cpp",