fix: Add or remove dune from package sets to allow resolving packages depending on it

[Leonidas-from-XIV]

There's multiple ways to go about it:

1. Add some dune package in the space of valid packages. I was not sure how to get the dune version out of dune_rpc or whether that would even be the right thing, so I set the version to dev.
2. Remove dune from the dependency formulas. Dependency_formula.remove_package formula (Package_name.Set.singleton Dune_dep.name) is an alternate solution.

Fixes #11096

[maiste]

One question to understand but it LGTM 👍

[Leonidas-from-XIV]

I've changed the PR from doing 1. to 2, as I didn't like how we construct a fake dune.dev (and I'd rather construct a dune.<current-version>); that said removing packages from formulas isn't particularly nice. I'm torn on what the best solution would be.

[rgrinberg]

We need to do a combination of 1 and 2. We need to pick the correct deps for a formula like ("a" "dune" {> 3}) | ("b" "dune" {< 3}).

So we give the right dune version when evaluating the formula and then filtering out dune from the evaluated list. We do the same for external deps as well.

I pushed something along those lines.