pkg: dune ignores the `avoid-version` flag in opam packages

[gridbugs]

This can lead to problems such as https://github.com/ocaml/dune/issues/10592 where the solver chooses development versions of packages.

There is an issue to address this upstream in opam-0install-solver (which dune vendors): https://github.com/ocaml-opam/opam-0install-solver/issues/23

I'm making this issue to document how this affects dune package management, though when we fix this we should upstream the fix and address the linked issue as well.

Reproduced in https://github.com/ocaml/dune/pull/10594 and https://github.com/ocaml/dune/pull/10595