p: add is_installed() method

check whether package(s) is/are installed in the chroot

based on an initial patch from Julien Cristau

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p: check the status of the packages after installing .debs

and fail if they are not installed

do not abort if installation of piuparts-depends-dummy.deb failed,
too many failures

Signed-off-by: Andreas Beckmann <anbe@debian.org>