Prefer local variants in preference selection

[charliermarsh]

Summary

This PR fixes a subtle issue arising from our propagation of preferences. When we resolve a fork, we take the solution from that fork and mark all the chosen versions as "preferred" as we move on to the next fork.

In this specific case, the resolver ended up solving a macOS-specific fork first, which led us to pick 2.6.0 rather than 2.6.0+cpu. This in itself is correct; but when we moved on to the next fork, we preferred 2.6.0 over 2.6.0+cpu, despite the fact that 2.6.0 only includes macOS wheel, and that branch was focused on Linux.

Now, in preferences, we prefer local variants (if they exist). If the local variant ends up not working, we'll presumedly backtrack to the base version anyway.

Closes #11406.