import: Avoid following symbolic links inside directories being imported

import: Avoid following symbolic links inside directories being imported

Too big a footgun.

This does not prevent attackers who can write to the directory being
imported from racing the check. But they can cause anything to be imported
anyway, so would be limited to getting the legacy import to follow into a
directory they do not write to, and move files out of it into the annex.
(The directory special remote does not have that problem since it does not
move files.)

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