Unclear how to model builds where different binary packages have different versions

While it's relatively rare, there are some source packages that use dpkg-gencontrol -v to give different versions to the different binary packages they build. The first example that comes to mind is gcc-defaults: currently in unstable, gcc is at version 4:13.2.0-2 while libgphobos-dev is at version 13.2.0-2.

It's not clear how this is supposed to be modelled in debusine. The current specification of debian:binary-packages says:

This artifact represents the set of binary packages (``.deb`` files and
similar) produced during the build of a source package for a given
architecture.
...
  * srcpkg-name: the name of the source package
  * srcpkg-version: the version of the source package
  * version: the version used for the build (can be different from the
    source version in case of binary-only rebuilds)

This says both that there's one ("the") set of binary packages for each build, and that there is a unique binary version. Both things can't be true.

Possible fixes might include:

• split the artifact into individual debian:binary-package artifacts (but the discussion in !427 (merged) seemed to prefer to avoid that idea)
• turn version into a dictionary in some way
• partition the artifact into one debian:binary-packages per unique binary version (but this feels like the worst of both worlds)