dgit: make $need_split_build_invocation always true

dgit: make $need_split_build_invocation always true

dgit will always build the source package.

One minor consequence is that dgit will usualliy generate
*_multi.changes rather than *_$arch.changes, so we need to update the
one test that makes the contrary assumption.

Bump the dgit major version number as this is quite a significant
change in implementation and also a behavioural change.

This change makes a lot of code dead.  Removing that is left as a task
for the future.

Signed-off-by: Sean Whitton <spwhitton@spwhitton.name>
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>