Make //:protobuf_python have correct __init__.py.

Previously //:protobuf_python set no __init__.py so Bazel created an
empty one. This change makes it use the __init__.py from the repository.

BUILD

@@ -693,6 +693,7 @@ py_proto_library(
         ":python_srcs",
         "//external:six",
     ],
+    py_extra_srcs = glob(["python/**/__init__.py"]),
     srcs_version = "PY2AND3",
     visibility = ["//visibility:public"],
 )