Generate field numbers for the well-known types,
so that encoding/jsonpb can benefit from them as well.
This CL fixes internal/cmd/generate-protos, which was silently failing
because the modulePath was not properly initialized. We fix this by
moving it to the start of the init function.
Change-Id: I87637176f29218cffa512b4baa49f39dae924061
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/168497
Reviewed-by: Herbie Ong <herbie@google.com>
