Reimplements the Git merge conflict resolution

[arcanis]

What's the problem this PR addresses?

The current merge conflict resolution algorithm is nice because it operates using only the conflict markers that Git puts into the files, but it has several drawbacks:

• Git sometimes generates bogus conflict markers, which lead to the resulting lockfile to have invalid checksums ([Bug] Automerge confused by change of checksums between yarn versions #2583, [Bug] yarn failed to handle merged yarn.lock with duplicated key #1771 (comment), ...)

• Git doesn't put conflict markers on the __metadata.version field (because it rarely change except when you upgrade Yarn, so there are no conflicts), which prevents us from knowing whether the merged lockfile has accurate metadata.

Closes #2583
Closes #5041

How did you fix it?

This diff reimplements the merge conflict auto-resolver to instead query the Git state. It retrieves the original variants of the lockfile, ignores the ones that don't make sense, tries to perform a couple of compatibility fixes if it knows how (useful for the lockfile v6 -> v7 migration), and merges the resulting data.

The main drawback is that the yarn install command must be run within the context of the conflicting git merge command. You cannot anymore accidentally commit the conflict, and fix it later (because then we'd lose the information telling us which commits are being merged).

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.