Cleanup the generated logic by having the implementation be backed
by protoimpl rather that directly generated.
Weak fields are a deprecated feature of protobufs and
have entirely be superceded by extensions.
Unfortunately, there are still some usages of it.
Change-Id: Ie1a4b7da253e2ccf5e56627775d9b2fb4090d59a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/229717
Reviewed-by: Damien Neil <dneil@google.com>