Improve marking packages as in-app by scanning classes in jar files

[NLthijs48]

Scan all plugin jars for .class files and mark those packages as in app (and maybe all super packages of them?). Already using a set for package names so duplicates will be discarded. This scan might take too much time, so it should be async.

This will improve this issue because the com.intellectualcrafters.plot... package would also get marked as in-app.