require explicit paths to dsc/control as we do for deb files

require explicit paths to dsc/control as we do for deb files

Otherwise a user is subject to unexpected content-injection depending on
which directory she happens to start apt in. This also cleans up the code
requiring less implementation details in build-dep which is always good.

Technically, this is an ABI break as we override virtual methods, but
that they weren't overridden was a mistake resulting in pure classes,
which shouldn't be pure, so they were unusable – and as they are new in
1.1 nobody is using them yet (and hopefully ever as they are borderline
implementation details).

Closes: 806693