Merging classes with nested classes fails with "Cannot compute access path" if anonymous package is used

Always, a SpoonException like this is thrown:

```
spoon.SpoonException: Cannot compute access path to type: Cls$CustomException in context of type: Cls
```

Start by reproducing the merge of classes with nested classes when an anonymous package is used, using the reported SpoonException as the failure point. Trace how the access path to Cls$CustomException is computed; done means the merge completes without the "Cannot compute access path" error.

Written by the indexing model from the issue text.