internal/impl: make resolverOnly explicitly implement file resolver
internal/impl: make resolverOnly explicitly implement file resolver

The resolverOnly type embeds a *protoregistry.Files and overrides one of
of its methods. When that method was renamed (Register -> RegisterFile),
the resolverOnly type continued to compile but no longer overrode the
desired method.

Update the method name, and change this type to explicitly forward
methods to the underlying *protoregistry.Files to catch errors of this
nature in the future.

Change-Id: I09a529034b6e2f310977bc635288fa15ce14cc7e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/205242
Reviewed-by: Joe Tsai <ad9e03fbec35106ba742f446a098eef3d5e31d20@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go