Fix most unhelpful conflict explanation message by merging all formulas together

[kit-ty-kate]

Fixes #4373

This PR fixes it by:

• using every available hints given by dose3 instead of just taking the first hint for a given set of packages (removal of ChainSet.precludes)
• merging all the extra information together

In some cases this makes the message as verbose but at least it is correct and helpful. We can try to make them even more readable in a later work but at least this fixes the biggest issue at hand: conflict messages telling lies and being unreadable.

~Built on top of https://github.com/ocaml/opam/pull/5210~ Also fixes https://github.com/ocaml/opam/issues/5422 (alcotest now only takes 45s to print the explanation instead of +infinit)

[rjbou]

related to #5130 too

[kit-ty-kate]

squashed