Support APT's Pre-Depends

[Jille]

Some installations currently fail (notably some Python stuff) because it uses Pre-Depends which dockpin doesn't handle correctly.

[johbo]

Just did bump into this while exploring how dockpin works. Found out that I can get by if the pre-dependency is installed in a first call.

In my experiments I got away by running apt-get install python3 in a step before executing dockpin. This is not a solution for a real world usage though.

It may be that it would be good enough to group the calls to dpkg -i so that the pre-dependencies are installed in a first call.