Bug#911651: apt: aptitude sync: add why command to apt for dependency chain detective work

[Paul Wise]

Package: apt
Version: 1.7.0
Severity: wishlist

It would be nice to have the why synced from aptitude.
It takes either one or two binary package names.

If launched with one package it discovers why it is installed by
looking at the auto/manual status of the package and the status of
other packages that depend on it and so on recursively until it finds a
manually installed package for the root of the dependency chain.

$ aptitude why gnome-shell
i gnome-shell-dbgsym Depends gnome-shell (= 3.30.1-2)

If launched with two packages it tries to discover a dependency chain
from the first package to the second package.

$ aptitude why gnome gnome-shell
i A gnome Depends gnome-core (= 1:3.22+11)
i A gnome-core Depends gnome-shell (>= 3.22)