LP#1942220: exclude invalid LIs when creating PO from SL

LP#1942220: exclude invalid LIs when creating PO from SL

LIs that are not in a valid state to be added to a PO (e.g.,
if they already were) are now excluded when creating a PO
from an SL. This fixes a problem where if a SL had some LIs
that were added to an order, including those LIs when creating
a new PO would cause the PO creation to silently fail.

Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>