Idiomatic import paths

## Feature Request

### Crates

`tonic_build`

### Proposal

For every `service ServiceName` definition, we add

```
pub mod service_name {
pub use super::{
service_name_client::ServiceNameClient,
service_name_server::{ServiceName, ServiceNameServer},
};
}

pub use service_name::ServiceName;
```

#### Pros

- Simpler, cleaner imports
- Naming follows `prost`. E.g. nested messages.

#### Cons

- Users need to use different imports (or we can just alias these and not make a breaking change)

### Motivation

Currently, we separate the client and server in two distinct modules which is not as idiomatic as just have one module that exports the client and server exports. For precedent, see [`std::path::Path`](https://doc.rust-lang.org/std/path/struct.Path.html).

### Alternatives

One can manually add these aliases.