Julian said that bash-completion files for apt-cache and apt-get located
in /usr/share/bash-completion/completions should be moved from the
bash-completion package to the apt package. Completions for the
relatively new apt command already reside with apt.

Implementation:
 * apt starts shipping the completions and gains versioned Replaces
   bash-completion.
 * bash-completion drops the completions. Depending on how important we
   consider completions, bash-completion can declare versioned Breaks on
   apt. Forcing an early upgrade of apt doesn't seem too bad.
 * The apt upload needs to be done first.

Helmut