cmd/protoc-gen-go: rename protoc-gen-go.go -> main.go

Name protoc-gen-go.go was temporarily used to keep git from getting
confused about the revision history of main.go.

Change-Id: I3b20ff93f750eaf54dcf3c5c3488d465fba5eaa1
Reviewed-on: https://go-review.googlesource.com/137036
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>