#302 (b) refactoring of MappingCreationProcessor: splitting into smaller creation concerns  

This is an alternative implementation to PR #303.
The `MappingResolverImpl` class has dependencies on both `org.mapstruct.ap.conversion`, `org.mapstruct.ap.model` `org.mapstruct.ap.model.assignment.Assignment`

If I were to move the `MappingResolverImpl` to  `org.mapstruct.ap.model`, that would result in cyclic dependencies between the packages, since `org.mapstruct.ap.conversion` is also using `org.mapstruct.ap.model.common`. 

So, apart from these dependencies issues, there's the underlying question were the mapping-resolver should be. After all, it resolves both conversions and mapping methods. So, its natural place would be close to the `MappingCreationProcessor`, perhaps even an inner class (one package for one class does not feel right either). 

I solved the issue by introducing an interface (IOC), in which the builders specify what they need from their creation environment.