The main apt completion seems to be shipped with the apt package, and apt-get and apt-cache with bash-completion.
In my opinion it would be best to ship all apt related completions with the apt package an deprecate/remove them from the bash-completion package. Please feel free to grab the apt-get and apt-cache completions from https://github.com/scop/bash-completion and let me know if/when you do, and I'll remove them from upstream bash-completion. There's also completion for apt-build shipped with bash-completion, BTW.