Replace Molinillo with PubGrub for dependency resolution

[mlarraz]

What was the end-user or developer problem that led to this PR?

My attempt at fixing #9365.

What is your fix for the problem, implemented in this PR?

• Vendor PubGrub from Bundler's existing copy
• Re-namespace it under Gem::PubGrub
• Rewrite Gem::Resolver to implement PubGrub's source interface
• Delete Molinillo

We should probably to de-duplicate the copies at some point but I figured that can wait.

Gem::Resolver now implements PubGrub's source interface (all_versions_for, versions_for, incompatibilities_for, no_versions_incompatibility_for) instead of Molinillo's interfaces.

Some notes:

• Version preference: all_versions_for returns highest-first, with already-installed versions preferred to avoid unnecessary upgrades. The skip_gems mechanism continues to work for conservative updates.
• Prereleases: Excluded from the preference index by default, but still available to the solver when a constraint requires them (e.g., = 2.a).
• Platform specificity: spec_for prefers InstalledSpecification when available, then selects by
  Gem::Platform.platform_specificity_match
• Error handling: A pre-check still raises UnsatisfiableDependencyError for root deps with zero matches. Everything else flows through PubGrub's SolveFailure -> DependencyResolutionError with its full explanation chain.
• Compatibility fix for InstallerSet: PubGrub needs all available versions upfront, but Source::Local#find_gem only returns the highest matching version. Added Source::Local#find_all_gems (returns all matches) and updated InstallerSet#find_all to use it.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[colby-swandale]

I appreciate the effort here, but I should flag that I was already actively working on this. I opened the issue, I'm assigned to it, and I've had a WIP implementation going for several weeks now.

That said, I don't want to be a gatekeeper, but I'm not exactly sure what the path forward here is.

[mlarraz]

I appreciate the effort here, but I should flag that I was already actively working on this. I opened the issue, I'm assigned to it, and I've had a WIP implementation going for several weeks now.

That said, I don't want to be a gatekeeper, but I'm not exactly sure what the path forward here is.

Makes sense. Not sure what your version looks like but feel free to use any ideas from mine if it helps. I would just like the change to get done.

[hsbt]

@colby-swandale Could you open a pull request with your current WIP branch? Even if it's still in progress, having it visible will help us coordinate.

Once your PR is up, we can compare both implementations and incorporate any useful differences from this PR into yours. The goal is to combine the best of both approaches into the final PubGrub integration.

[colby-swandale]

Lots of progress over the weekend. Performance is noticeably much much improved, which is encouraging.

That said, there's still a fair amount of resolver behaviour to verify and work through before this is ready. Will block out some more time this week before RubyKaigi to get this ready for review.