Bug#825034: debootstrap: fails if no base packages are to be installed

[Ansgar Burchardt <ansgar@debian.org>]

Package: debootstrap
Version: 1.0.67
Severity: normal

Helmut Grohne reported in #-devel that `debootstrap --variant=minbase`
fails for unstable:

+---
| Setting up systemd-sysv (229-6) ...
| Setting up init (1.33) ...
| Processing triggers for libc-bin (2.22-9) ...
| dpkg: error: --unpack needs at least one package archive file argument
|
| Type dpkg --help for help about installing and deinstalling packages [*];
| Use 'apt' or 'aptitude' for user-friendly package management;
| Type dpkg -Dhelp for a list of dpkg debug flag values;
| Type dpkg --force-help for a list of forcing options;
| Type dpkg-deb --help for help about manipulating *.deb files;
|
| Options marked [*] produce a lot of output - pipe it through 'less' or 'more' !
+---

This is probably caused by "apt"'s priority getting increased from
"important" to "required".  The "minbase" variant then installs no
additional base packages (base="apt" is already installed).

Helmut confirmed that adding another packages ("ed") as an additional
base package to the "minbase" variant made `debootstrap` complete
successfully.

I've reverted the priority of "apt" back to "important" for now.

Ansgar