Break Gradle plugin out of pbj-core and pbj-compiler

The PBJ project has a funky build due to the fact that it builds a plugin, and that plugin is part of the pbj-compiler project. I think we could instead rearrange the project such that:

```
root
|
- pbj-compiler
- pbj-runtime
- pbj-gradle-plugin
- pbj-grpc-helidon
- tests
- pbj-integration-tests
```

The compiler depends on the runtime, the gradle plugin depends on both, and the helidon plugin depends on all three. There would be a single multi-project gradle build, and each of the above would be a different module.