We'd like to prune agressively
https://github.com/istio/istio/blob/master/Gopkg.toml#L41-L44

yet we have a couple of directories we do need to keep some non go files (specifically .proto files)

I was expecting adding those to required= would do the trick but it doesn't appear to

https://github.com/istio/fortio/blob/master/fgrpc/ping.proto is missing
from
https://github.com/istio/vendor-istio/tree/master/istio.io/fortio/fgrpc
despite
https://github.com/istio/istio/blob/master/Gopkg.toml#L37

(or is it I should add istio.io/fortio/fgrpc too ? I will test that... but in general having the ability to scope the pruning to a constraint would be great)