Refactor the python system_impl module.

mojo/public/python/BUILD.gn

Index: mojo/public/python/BUILD.gn
diff --git a/mojo/public/python/BUILD.gn b/mojo/public/python/BUILD.gn
index 8f10250dc1ab0270474788fda61ab507bcd2355b..ab0ef22ad5d52975f6d3b86984da331567601377 100644
--- a/mojo/public/python/BUILD.gn
+++ b/mojo/public/python/BUILD.gn
@@ -16,16 +16,22 @@ group("python") {
 # 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",
@@ -34,18 +40,18 @@ python_binary_module("system") {
 
 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",
@@ -55,6 +61,20 @@ python_binary_module("system_impl") {
   ]
 }
 
+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",