sbuild: Pin experimental extras with --add-depends

[aquilamacedo]

When building with experimental we follow the buildd aspcud criteria:
-count(down),-count(changed,APT-Release:=/experimental/),-removed,-changed,-new

With these criteria, aspcud may still choose dependencies from unstable even if we pass locally rebuilt experimental .deb via --extra-package. In that case, reverse-dependency builds end up using the in-archive versions instead of the .deb we are trying to test, which can hide regressions (see:
https://salsa.debian.org/salsa-ci-team/pipeline/-/issues/519).

When experimental and extra .deb's are provided, add each as an exact build-dependency:
--add-depends=pkg (= version)
so the solver installs the intended experimental versions from experimental instead of mixing in unstable.

[aquilamacedo]

I also considered other options (like tweaking the aspcud criteria or using apt
pinning), but TBH this seemed like the most practical solution that stays
self contained in ratt/sbuild.

The main downside is that the sbuild command line grows with the number of
extra binaries, since each one becomes an --add-depends entry. I don't expect
this to be a big problem in practice, but if you or @ottok or @stapelberg
see a cleaner way to ensure the extra experimental .deb files are always used,
I'm happy to rework it.

It would be useful if sbuild had a native option to install specific .deb files
directly in the chroot without running them through the dependency solver. That
would help avoid overly long sbuild command lines when a source package
produces many binary .deb files.

[rbalint]

@aquilamacedo IMO this will break builds when the produced binary packages are in conflict. I think the best way would be going with pinning.
Also simply adding dependencies could make the tested reverse dependency packages build in unexpected ways.