Adds legacy plugin removal behavior

[arcanis]

What's the problem this PR addresses?

When using yarn set version with Yarn 4, the core plugins that were previously externals now conflict with the builtin ones, causing surprising errors.

Fixes #4952

How did you fix it?

Yarn will now ignore the specified list of plugins when booting. It'll then automatically remove them during the next install.

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.

[arcanis]

I reverted the support for immutable installs. I'd still like to support it, but I didn't find a way that didn't feel overkill. Things I considered / tried:

• Make a ReadOnlyFS that would throw on FS access (but --immutable allows FS access - it just throws if the result is different from the initial state, so for example a recursive mkdir would work if the folder already exists).
• Add writeFile / readFile callbacks to the changeFile functions (but that seemed redundant with the FakeFS interface itself).
• Wrap xfs in updateConfiguration using Object.create, and override its writeFilePromise implementation (but that seemed to leak that changeFilePromise uses writeFilePromise under the hood).
• Inline the code for changeFilePromise (but since it does some magic around line endings, it didn't feel right to include this code inside updateConfiguration).

One of those might still work in the end, but for this iteration I think it's reasonable to ship the main fix and think more about the --immutable situation.