Make "duplicate" package-info.java files symbolic links
This is needed for javadocAll since otherwhise there will be
smack-core/src/main/java/org/jivesoftware/smack/package-info.java:21:
warning: a package-info.java file has already been seen for
package org.jivesoftware.smack
travis: Add javadocAll to 'script' gradle targets
to ensure that javadocAll works correctly. A nice side-effect, this
also ensures that the package-info.java symbolic links are still
correct and haven't been replaced by some IDE or editor with the
content of the link target.
