Disallow inner classes

[jayasuryat]

From what I could gather, there is no meaningful scenario where objects of inner classes could be used in constructors of other classes.

It would be better to detect usages of inner classes annotated with @Dowel annotation and log an error with a meaningful error.

[jayasuryat]

The same also applies for @ConsiderForDowel, so need to add the same checks for @ConsiderForDowel as well.

@DowelList is already covered as it can only be applied to classes already annotated with @Dowel annotation. So all of the checks already added for @Dowel should be sufficient to provide necessary information or errors.