The genid package unifies the genname, fieldnum, and detectknown
packages into a single package.
Whenever possible use the generated constants rather than
hard-coded literals. This makes it easier to search the entire
module for special logic that deal with well-known types.
Change-Id: I13beff1f4149444a0c0b9e607ebf759657f000f4
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/235301
Reviewed-by: Herbie Ong <herbie@google.com>
