Support classes that were not generated by dor_gen

[michalowskim]

Currently dor_gen supports only classes that were generated by itself.
Let's see example below

@Dto()
class A {
  final B b;

  A(this.b);
}

class B {}

You can't include B as a field in A - dor_gen require B to be generated by dor_gen - it will look for this class in ./b.dto.g.dart but in fact is is somewhere else.

I think we should somehow support classes from outside of dor_gen, so they can be used and combined with each other.