Packr 3.0.0 several fixes

1. Refactored code to fit better into libGdx/packr parent repository.
2. Fixed an issue where extracting an archive with duplicate entries would fail.
3. The packr-all Jar is available from GitHub packages https://github.com/libgdx/packr/packages.
4. The output directory specified by --output must be an empty directory, or a path that does not exist.
   • Packr will no longer delete the output directory and then populate it.
5. Packr will ensure that jspawnhelper has execute permissions in the bundled JRE.
   • This resolves an issue on macOS where Runtime.exec and Process.start calls would fail with java.io.IOException: Cannot run program "java": error=2, No such file or directory.
     • Reference: https://stackoverflow.com/questions/53113127/java-runtime-exec-fails-to-run/55091040
6. Updated ArchiveUtils to support POSIX permissions and symbolic links.
   • This resolves an issue with repackaged JREs losing execute permissions.
7. Alternative Maven repositories until oss.sonatype.org (Maven central) publishing is possible.
   • Snapshots: http://artifactory.nimblygames.com/artifactory/ng-public-snapshot/
   • Releases: http://artifactory.nimblygames.com/artifactory/ng-public-release/