check copyright/license headers when importing new upstream

It would be great if we could scan new files for copyright/license headers when importing new upstream versions. Ideally we'd just scan the diff between the old and the new upstream and then verify that relevant copyright/license entries are already present in debian/copyright.

If not, a first step would be to make the user aware of that. Even better would be to actually propose changes to debian/copyright.

Usual caveats apply - we can make this easier, but a human should still verify everything.

decopy and https://github.com/nexB/scancode-toolkit provide a good base for this.