Package protogen provides support for writing protoc plugins.
A "plugin" in this case is a program run by protoc to generate output.
The protoc-gen-go command is a protoc plugin to generate Go code.
cmd/protoc-gen-go/golden_test.go is mostly a straight copy from
the golden test in github.com/golang/protobuf.
Change-Id: I332d0df1e4b60bb8cd926320b8721e16b99a4b71
Reviewed-on: https://go-review.googlesource.com/130175
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
