do not restore selections for already purged packages

do not restore selections for already purged packages

In most cases apt was already skipping the (re)setting of packages as
to be removed/purged if dpkg had told us that it already did, but we
haven't dealt with it in the most obvious of the cases: Selections set
for packages we touched in this operation which either restores
selections even dpkg would have overridden or e.g. tries to restore a
purge selection for a package which was just purged – does not happen
with apt itself as it isn't using selections in this way, but higher
frontends like aptitude do.

The result in the later case is a warning printed by dpkg that we try to
set selections for an unknown package, which is harmless per se, but can
be confusing for users and we really shouldn't cause warnings in dpkg if
we can help it.

Reported-By: Guillem Jover on IRC