We can only unveil if the prefix is a directory (the input paths, and the

We can only unveil if the prefix is a directory (the input paths, and the
output directory).  If prefix isn't a directory, that would require
enumerating all prefix<sig>.<id> filenames and unveiling all of them
which isn't reasonable... for the file case can we identify whether it
starts start with '/' or not, and unveil '/' or '.' for "w"?