Removal of the package index files, aka lists, is something that needs
to be done explicitly. Doing so at `APT::Update::Post-Invoke` defeats
the purpose of running `apt-get update` in the first place.
This integrates a custom script into the APT configuration. That same
script is also run during the bootstrap. A second custom script takes
care of the integration and removes itself when done.
Complements 13f84159.
Olaf Meeuwissen