Add support for Java modules

[clausnagel]

Any plans to turn classindex into a Java module?

[sentinelt]

I don't have any plans for this, as I don't see much benefit in it. I will gladly accept pull request if anyone needs this and is willing to create a pull request.

[clausnagel]

I have done the exercise in a fork: https://github.com/xmlobjects/classindex. But I also moved to Gradle and require at minimum Java 11 in this fork. So, not sure whether this can serve as template for this repository. Anyway, I can go on with my fork in the meantime.

[sentinelt]

@clausnagel Could you pinpoint to me which changes are required only for Java module? Is module-info.java sufficient?