Fix a dependency loop

Fix a dependency loop

When generating units with zfs-mount-generator, if the pool is already
imported, zfs-import.target is not needed.  This avoids a dependency
loop on root-on-ZFS systems:
  systemd-random-seed.service After (via RequiresMountsFor)
  var-lib.mount After
  zfs-import.target After
  zfs-import-{cache,scan}.service After
  cryptsetup.service After
  systemd-random-seed.service

Reviewed-by: Antonio Russo <antonio.e.russo@gmail.com>
Reviewed-by: InsanePrawn <insane.prawny@gmail.com>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #10388