Break the circular dependency in the new mojom bindings generation pipeline.

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 5fe296cea696941cfa563cad6e6172e6ec4f4372..01de7f45dbcbdc396dc6b96d72fe27878879f084 100644
--- a/mojo/public/python/BUILD.gn
+++ b/mojo/public/python/BUILD.gn
@@ -115,7 +115,16 @@ action("import_interface_bindings") {
   deps = mojom_deps
 }
 
-copy("bindings") {
+group("bindings") {
+  deps = [
+    ":import_interface_bindings",
+    ":serialization_bindings",
+  ]
+}
+
+# This target lists dependencies needed to use generated mojom files for
+# serialization/deserialization only, but not the full mojo application stack.
+copy("serialization_bindings") {
   sources = [
     "mojo_bindings/__init__.py",
     "mojo_bindings/descriptor.py",
@@ -129,7 +138,6 @@ copy("bindings") {
     "$root_out_dir/python/mojo_bindings/{{source_file_part}}",
   ]
   deps = [
-    ":import_interface_bindings",
     ":mojo_system",
   ]
 }