Support generating ISOs when using link_type="symlink".
    
    When `link_type = "symlink"` is used, the packages are in fact symlinks
    to /mnt/koji. When graft points file is generated, the paths in this graft
    points file point to symlinks and therefore symlinks are copied into the
    generated ISO file instead of real files.
    
    In this commit, the code to generate the graft points file is changed
    so it resolves the symlink to real file stored on /mnt/koji. To make
    this code safer, it does such resolving only in case the symlink points
    outside of `compose.paths.compose.topdir()`. Therefore you can still
    generate ISO file with symlink pointing to file stored within the ISO
    file itself, although this is not done currently afaik.
    
    The main reason for this is to be able to generate ISO files even
    without hardlinks (which would need read-write access on /mnt/koji)
    and without copying all the packages from /mnt/koji to local storage.
    
    Signed-off-by: Jan Kaluza <jkaluza@redhat.com>