Bring back `.solv` files for libsolv

[wolfv]

The switch to resolvo made it so that we don't write .solv files anymore. They help quite a bit with speed for libsolv though, so it might be nice to bring them back. :)

[benmoss]

I tried starting on this, but I got lost in trying to make the AvailablePackageCache cache SolverRepoData (the trait).. Maybe that's not the right approach, but I'm too new to Rust to figure out how to satisfy the compiler (if it's even possible)