dgit: Check that entirely-new uploads to Debian are not source-only-uploads.

Avoids the user pushing things that will inevitably be REJECTed.

We end up introducing some new config and force options to support
this.

We do not reuse test_source_only_changes, as it doesn't have quite the
right shape (in particular, it sometimes blithers on stdout).  Also
arguably it is better to look, specifically, for .debs, for the
purpose of this test.

This requires a new archive protocol method.  We implement it for
ftpmasterapi (where it is actualliy needed) and dummycatapi (where we
need it for tests.)  Implementing it for madisonish methods would be
easy.  For aptget, it would probably involve iterating over suites,
and not be adviseable.

Closes:#801435.

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