packages: Fix rpm generation

The root problem was that the --transform tar option would by default apply the prefix on regular files AND links. This would result in the links targetting packagename-version/libfoo.so instead of just libfoo.so

This would also exhibit another issue with tar usage, which is that when you provide it with several times the same file, it would make hardlinks of those duplicate files ... which would result in symbolic links becoming hard links. Instead of that, ensure that the file list we provide to tar is unique.

See #154