What's the problem this PR addresses?

Fixes #3317

How did you fix it?

I'll write in more details tomorrow, but in a few words:

• Yarn will only download by default packages relevant to the current system.
• More packages can be "overfetched" for zero-install purposes via the supportedArchitectures setting.
• Optional peer dependencies don't have their checksum computed nor stored.
• The point above isn't a security risk because --check-cache will still validate them, for zero-install users.
• When stored inside the offline mirror, their paths use cache keys, not checksum keys (since we don't keep the checksum).
• They will always be unplugged when installed with PnP (the assumption being that they contain native bins anyway).
• Transitive dependencies of optional native dependencies are always fetched (unless themselves have os/cpu markers).

Note that while it contains breaking changes, I plan to land it as a minor. This is due to multiple factors:

• Most changes are on the internal API, which aren't used that much and still a little experimental.
• Most changes in those API are fairly small (extra fields, a few parameters on the cache), shouldn't break much.
• For a significant segment of our users, this would appear as a bugfix more than an improvement.
• I don't want to release a 4.0 just for the sake of this one thing.

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.