dgit import: Right error message for missing files in ..

Close examination of this code path reveals that:
 * The error is generated only if $there contains no slash.
 * This can only occur if $dscfn matches the first regexp,
   ie $dscfn is [./]../X in which case $there becomes X
 * So in this situation, $there is simply the dsc filename
   which is supposed to be in ..
 * What we should be testing is ../$f but that is $here
   which is what are trying to create and which we statted
   earlier and got ENOENT for.

So this occurs when the dsc is in .. and a file it refers to is
missing.

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