Refactor the python system_impl module.

mojo/public/python/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//third_party/cython/rules.gni")
 
 group("python") {
   deps = [
     ":base",
     ":bindings",
     ":system",
     ":system_impl",
   ]
 }
 
 # GYP version: mojo.gyp:mojo_python_system
 python_binary_module("system") {
   python_base_module = "mojo"
-  sources = [
+  configs = [ "../build/config:mojo_sdk" ]
+  deps = [
+    ":system_embedded",
+  ]
+}
+
+python_binary_source_set("system_embedded") {
+  cython_sources = [
     "mojo/c_core.pxd",
+    "mojo/c_async_waiter.pxd",
+    "mojo/c_thunks.pxd",
     "mojo/system.pyx",
   ]
-  configs = [ "../build/config:mojo_sdk" ]
   deps = [
     ":base",
-    "../c/environment",
     "../c/system:for_shared_library",
-    "../cpp/environment:standalone",
     "../cpp/system",
     "../cpp/utility",
     "../cpp/bindings:callback",
   ]
 }
 
 python_binary_module("system_impl") {
   python_base_module = "mojo"
-  sources = [
-    "mojo/c_core.pxd",
+  cython_sources = [
+    "mojo/c_thunks.pxd",
     "mojo/c_environment.pxd",
     "mojo/system_impl.pyx",
   ]
-  additional_sources = [
+  sources = [
     "src/python_system_helper.cc",
     "src/python_system_helper.h",
   ]
-  configs = [ "../build/config:mojo_sdk" ]
   deps = [
     ":base",
+    ":python_common",
     "../c/environment",
     "../c/system:for_shared_library",
     "../cpp/environment:standalone",
     "../cpp/system",
     "../cpp/utility",
     "../cpp/bindings:callback",
   ]
 }
 
+python_binary_source_set("python_common") {
+  sources = [
+    "src/common.h",
+    "src/common.cc",
+  ]
+  deps = [
+    "../c/environment:environment",
+    "../cpp/bindings:callback",
+    "../cpp/environment:environment",
+    "../cpp/system:system",
+    "../cpp/utility",
+  ]
+}
+
 copy("base") {
   sources = [
     "mojo/__init__.py",
   ]
   outputs = [
     "$root_out_dir/python/mojo/{{source_file_part}}",
   ]
 }
 
 # GYP version: mojo.gyp:mojo_python_bindings
 copy("bindings") {
   sources = [
     "mojo/bindings/__init__.py",
     "mojo/bindings/descriptor.py",
     "mojo/bindings/messaging.py",
     "mojo/bindings/promise.py",
     "mojo/bindings/reflection.py",
     "mojo/bindings/serialization.py",
   ]
   outputs = [
     "$root_out_dir/python/mojo/bindings/{{source_file_part}}",
   ]
   deps = [
     ":base",
     ":system",
   ]
 }