support/gen-manual-lists.py: bail out early for legacy symbols

When searching for virtual package providers, there's no need to
handle legacy symbols at all, so just bail out early.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>