Fixed bugs

• Restrict getOutputSourceDirectorySet() to directories only (#533). This fixes build failures for certain configurations when outputPath is a zip/jar

Improvements

• Ignore empty directories for sources (#530). This is in preparation for Gradle 8 which will stop implicitly skipping the task when all directories are empty
• Clean up Kotlin integration using newer Gradle integration (#531). Does not change minimum Gradle version supported

Thanks to contributors

• @bigdaz
• @cilki
• @wolfs