dgit import: Avoid making broken symlinks in ..

dgit import: Avoid making broken symlinks in ..

In most cases we could carry on and fail later.  But creating a broken
symlink is undesirable, particularly because it might prevent dgit
from trying to make a non-broken symlink pointing elsewhere in future
(or prevent dget from downloading the file).

Even worse, if the dsc is in .. but an absolute path was provided, we
would make a circular symlink!

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>