Add check to prevent exposing internal types in public API

[gunnarmorling]

When declaring the public API of a Java module in module-info.java, the Java compiler doesn't prevent exposure of any non-public types in the API signatures. That situation should be detected by Deptective and reported accordingly.

[gunnarmorling]

@nilshartmann, @wuetherich, just realizing this interesting use case for Deptective.

[gunnarmorling]

For applications without a module-info.java, the information of what constitutes the public API of the current compilation should be taken from deptective.json.