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

[gridbugs]

This can lead to problems such as #10592 where the solver chooses development versions of packages.

There is an issue to address this upstream in opam-0install-solver (which dune vendors): ocaml-opam/opam-0install-solver#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 #10594 and #10595