internal/filedesc: use genid constants for map entry fields
internal/filedesc: use genid constants for map entry fields

Change-Id: If551fc4f3e8da6325af7a0125f0e96fb014d3636
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/238998
Reviewed-by: Herbie Ong <86f376bf42b6daa37c768c730acd318ab6855de4@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go