dpkg-maintscript-helper: Fix dir_to_symlink to handle relative symlink targets

When invoking dir_to_symlink, dpkg-maintscript-helper was assuming the
target to be absolute, but Debian policy 10.5 requires relative symlinks
in a few places. So it now works with relative targets.

Signed-off-by: Helmut Grohne <helmut@subdivi.de>
Signed-off-by: Guillem Jover <guillem@debian.org>