breaking the egg / hatching the chicken

[v4hn]

A bunch of modules have the traditional chicken-and-egg problem, where modules depend on each other, e.g., libvnd and mesa-lib.

While the general problem cannot be solved without providing binaries for either package, we could handle cycles with optional_depends automatically.

Once such a cycle is detected (this doesn't happen at the moment),
the system could automatically disable a specific optional dependency to break the cycle, compile the packages and in the end compile the first package again, enabling the dependency.

To designate a specific dependency for this process, I'd propose to use a new breakable_optional_depends function.

Still, implementing this is tricky, especially because we have to consider the case where any package-build in between fails.