Boa pergunta. Tanto quanto eu sei, não há maneira de descobrir onde um pacote foi instalado depois de remover a linha de deb de você sources.list -file.
A única maneira de salvar todas as fontes seria algo assim:
% grep -h -e "^.*deb " /etc/apt/sources.list /etc/apt/sources.list.d/* | sort | uniq
deb http://apt.spideroak.com/ubuntu-spideroak-hardy/ release restricted
deb http://APT.spideroak.com/ubuntu-spideroak-hardy/ release restricted
deb http://deb.grml.org/ grml-stable main
deb http://debian.lagis.at/debian/ jessie main contrib non-free
deb http://debian.lagis.at/debian/ jessie-updates main contrib non-free
deb http://download.virtualbox.org/virtualbox/debian jessie contrib
deb http://extra.linuxmint.com betsy main
deb http://http.debian.net/debian jessie-backports main
deb http://http.debian.net/debian stretch main contrib non-free
deb http://mirror.karneval.cz/pub/linux/linuxmint/packages betsy main upstream import
deb http://repo.linrunner.de/debian jessie main
deb http://repository.spotify.com stable non-free
deb http://security.debian.org/ jessie/updates main contrib non-free
deb http://www.deb-multimedia.org jessie main non-free
Para confirmar minha declaração, tentei fazer isso:
% apt-cache policy | sed -n -e "/^.[0-9]/p" | cut -f3,4 -d' ' | sort | uniq
http://apt.spideroak.com/ubuntu-spideroak-hardy/ release/restricted
http://APT.spideroak.com/ubuntu-spideroak-hardy/ release/restricted
http://deb.grml.org/ grml-stable/main
http://debian.lagis.at/debian/ jessie/contrib
http://debian.lagis.at/debian/ jessie/main
http://debian.lagis.at/debian/ jessie/non-free
http://debian.lagis.at/debian/ jessie-updates/contrib
http://debian.lagis.at/debian/ jessie-updates/main
http://debian.lagis.at/debian/ jessie-updates/non-free
http://download.virtualbox.org/virtualbox/debian/ jessie/contrib
http://extra.linuxmint.com/ betsy/main
http://http.debian.net/debian/ jessie-backports/main
http://mirror.karneval.cz/pub/linux/linuxmint/packages/ betsy/import
http://mirror.karneval.cz/pub/linux/linuxmint/packages/ betsy/main
http://mirror.karneval.cz/pub/linux/linuxmint/packages/ betsy/upstream
http://repo.linrunner.de/debian/ jessie/main
http://repository.spotify.com/ stable/non-free
http://security.debian.org/ jessie/updates/contrib
http://security.debian.org/ jessie/updates/main
http://security.debian.org/ jessie/updates/non-free
http://www.deb-multimedia.org/ jessie/main
http://www.deb-multimedia.org/ jessie/non-free
/var/lib/dpkg/status
que basicamente é o mesmo.