Fix for massfords/jaxb-visitor issue #4.

[unserializable]

This prevents #4 (methods with duplicate signatures) from occurring. Cause of duplication remains unclear to me.

[massfords]

@unserializable - Check out the new branch named issue-4.

I used your XSD as a test case and saw that due to its nested types some of the generated classes were mistakenly being identified as external classes. This is why they were duplicated. I fixed the check in the discovery code.

If you're ok with having your XSD as a test case then I'll merge this branch over and cut a release.

[unserializable]

Hey @massfords, thanks for the quick reaction!

I checked out issue-4 branch and found it working as expected with regard to inner static classes.

Feel very free to use that particular XSD, which is slightly modified "state provided publicly available XML resource without any usage restrictions" from https://riha.eesti.ee/riha/main/xml/oigusakt