Consider pruning unused imports

[lucasdicioccio]

Hi all. First of all great job with proto-lens. I'm pondering whether some improvement is possible to prevent proto-lens from generating unused imports (from what I've seen, they are often due to option-plugins imports). I've not enough hindsight to understand if such a pruning step would be feasible and correct in general.

For the context, I recently built and published a minimal etcd-v3 gRPC client (cf. https://hackage.haskell.org/package/grpc-api-etcd ). One minor annoyance is the need to generate Haskell code for some un-used annotations (e.g., Google.Api.Http -- which has no incidence on proto-lens code at runtime). The alternatives are: modify the .proto or modify the generated .hs code, both of which will bring more problems down the road.