apt manpage restructuring

[Julian Andres Klode <jak@debian.org>]

Currently, we have one manual page per executable, with apt(8)
saying to look at apt-get(8) and apt-cache(8) for options and
longer descriptions of commands.

I'd like us to move to per-command manual pages, so you can do

  man apt install
  man apt update

etc (the manpages being called apt-install, apt-update, apt-get-update,
and so on).

One thing to consider is how to handle "apt install" and friends;
we could create one manpage apt-install -- and then links apt-get-install,
{apt,apt-get}-{reinstall,remove,purge,{dist-,safe-,full-,}upgrade,autoremove,autopurge},
so we capture all commands managing packages, taking a list of packages as an argument. 

They are essentially the same code paths, so it makes sense to have them in the same
manual page, and describe their differences, rather than have individual pages linking
to each other?

aliases:
* apt reinstall is the same as apt install --reinstall
* apt purge is the same as apt remove --purge
* apt autopurge is the same as apt autoremove --purge
* apt dist-upgrade is apt full-upgrade
* apt upgrade is apt safe-upgrade

behaviour differences:
* apt remove removes packages by default
* apt full-upgrade first marks all packages for upgrade, and removes as needed
* apt safe-upgrade is like full-upgrade, but does not remove

Apart from those, they _should_ all be taking the same arguments, so
splitting them into separate manual pages would require explaining things
multiple times.
-- 
debian developer - deb.li/jak | jak-linux.org - free software dev
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