Refactor the code to only separate the download bits

The script is designed to, on-the-fly, download packages from Debian and import it in the format that we maintain for Apertis.

But when we have a package which is not in Debian, just passing a dsc to this script with option --import-local-dsc is helpful.

This change just refactors the code to keep the commond code consistent for both the cases.

Signed-off-by: Ritesh Raj Sarraf ritesh.sarraf@collabora.com