Too much code inside Google rely on ExtensionDescV1.Name being the
target message name rather than the real name of the extension.
Double down on this assumption consistently across our codebase.
Change-Id: If0b7cd16dd1f2f7acfb2e562dbf1a2b5487162b5
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/186980
Reviewed-by: Herbie Ong <herbie@google.com>