apt-get wrapper: purge and remove actions can trigger the installation (and thus, the download) of packages.

As noticed by anonym (https://labs.riseup.net/code/issues/10748#note-32),
lockfile-progs is missing from the build manifest.

Indeed, it is installed by a call to `apt-get purge', found in
config/chroot_local-hooks/98-remove_unwanted_packages, whose effect is:

The following extra packages will be installed:
  lockfile-progs
The following packages will be REMOVED:
  apt-listchanges* at* autoconf* automake* bsd-mailx* dc* debian-faq*
  doc-debian* exim4* exim4-base* exim4-config* exim4-daemon-light* ftp* m4*
  mlocate* mutt* ncurses-term* nfs-common* procmail* python-reportbug*
  reportbug* telnet* texinfo* time* w3m* wamerican*
The following NEW packages will be installed:
  lockfile-progs

For the curious: monkeysphere "Depends: lockfile-progs | procmail", so
when we remove procmail, apt-get installs lockfile-progs to satisfy
the dependency.