Add package cycle check

[gunnarmorling]

While Deptective already implicitly addresses the detection of package cycles by validating a code base against a target architecture (provided that architecture is defined in a cycle-free way), there's demand (One example being the JUnit team) for ready-made cycle detection out of the box. I think that's a reasonable addition.

[gunnarmorling]

@nilshartmann @wuetherich, another thing to discuss tomorrow night. Perhaps @sormuras from the JUnit team would even be interested in providing his existing routine to Deptective, retrofitted to work with the parser's AST API?

[sormuras]

Sure. Count me in.

[gunnarmorling]

Thanks, that's much appreciated! As it turns out, @wuetherich is working on a PR already, so it's probably solved very soon. But in any case we'd love your contributions to other loose ends in Deptective.