Simplify generator logic

[ioleo]

Currently some processes are unnecessarily complicated. We're getting a admingenerator.generator.doctrine service, which is injected with some stuff.. only to pass it to other classes, which are instantiated with new Classname.

Instead, we should create new (non public) services for these classes and inject the right dependencies directly where they are needed (eg. into builders).

This should clean up much of the duplicated code and simplify the bundle internals.

[ioleo]

Create a dedicated class with static method getFromArray and use it across all classes (reduce code duplication).