Refactor the internal logic of protoc-gen-go to better plumb local
settings and parameters down the call tree.
Change-Id: I09fec188d7359f2b66be584aa8f10e682a7b6796
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/214357
Reviewed-by: Patrik Nyblom <pnyb@google.com>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
