reflect/protoregistry: panic upon registration conflicts
reflect/protoregistry: panic upon registration conflicts

In this CL, we switch the behavior of registration conflicts
from simply logging a warning to panicking.
This is a change that we have warned about ever since
the release of v1.20.0 in 2020-03-02.

For users that have not fixed their dependencies or are unable to,
they may set the GOLANG_PROTOBUF_REGISTRATION_CONFLICT environment
to "warn" in order to obtain the prior behavior of simply logging
and ignoring subsequent conflicts (first wins).

Change-Id: Ied7937f0fd5cb5d875858411714ebeb5ce145574
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/235298
Trust: Joe Tsai <ad9e03fbec35106ba742f446a098eef3d5e31d20@digital-static.net>
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go