hidden support more apt-get/apt-cache commands in apt

hidden support more apt-get/apt-cache commands in apt

apt is supposed to be a user-friendly interface, so while these commands
are usually poweruser material and therefore do not need to be shown in
general introduction manpages/help messages its of no use to not allow
users to use them.

This includes clean, autoclean, build-dep, source, download, changelog,
depends, rdepends and showsrc – it doesn't include more non-interactive
commands like dump or xvcg as those are usually used by scripts if at
all.

Closes: 778234, 780700, 781237