Draft: build-needed: add disambiguating Conflicts: to built packages
When autopkgtest builds packages, we want to make sure that the built packages are used on the testbed, even if they have the same version those available from the archive, which are possibly already installed.
APT can get confused by this situation, especially after the changes introduced to fix #931175.
Here we artificially inject a Conflicts: against a nonexisting package to make sure APT can differentiate packages regardless of the version.
Closes: #1086701