Better support for optional dependencies

Right now it’s awkward to install a dependency using runtime information:

```
  // TODO(jwilson): we should design a better mechanism to select dependencies.
  val tracingModule = if (serviceConfig.zipkin_tracing != null) {
    ZipkinTracingModule(serviceConfig.zipkin_tracing)
  } else {
    NoopTracingModule()
  }
```

We should design something better.