protoregistry: avoid checking for '/' in FindMessageByName
protoregistry: avoid checking for '/' in FindMessageByName

Profiling shows FindMessageByName spending considerable time
in strings.LastIndexByte in FindMessageByURL.

This overhead can be avoided by giving FindMessageByName
its own implementation not pruning the name string up
to the last '/' which can not appear in a protobuf name.

Change-Id: Ifec07689f72790ead8b7552cd04dd99fa67b61e4
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/240378
Reviewed-by: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go