Support gRPC packages in generated code

Include package names in the `NamedService` identifier and request
paths. Change generated imports to use `crate` and add `tokio` and
`tokio-stream` dependencies to `roto-tonic`.

roto-tonic/src/lib.rs

@@ -8,6 +8,10 @@ use std::future::Future;
 use std::task::{Context, Poll};
 use http_body::Body;
 
+pub mod generated {
+    pub mod helloworld;
+}
+
 pub struct RotoCodec<T, U> {
     _phantom: PhantomData<(T, U)>,
 }