🐘 Gradle config: mandate service name

[martinbonnin]

In version 3.7.0 and below, it is possible to use a default service:

apollo {
  packageName.set("com.example")
}

While this is working and easy, this has 2 drawbacks:

1. it forces one afterEvaluate {} in the Gradle plugin code which makes the behaviour reliant on the ordering of other plugins and is error prone
2. as we move to more build time codegen, we will generate code based on the service name so making the user aware of that name will make things more explicit and less magic (see Automatic adapters registration #4515)

[martinbonnin]

Done in main