feat(pack): Allow temporary changing the package.json during prepack

[arcanis]

What's the problem this PR addresses?

Making changes to the package.json during prepack is annoying, because whatever changes are made also need to be reverted (or committed, but if they are applied during build it's likely that the users don't want them).

How did you fix it?

This diff implements a new environment variable when the prepack lifecycle script is called: PACK_MANIFEST. If this file exists after prepack returns, Yarn will read it and merge it to the final manifest used to pack the package, before automatically removing it from the filesystem once it's no longer needed.

This way, prepack scripts have the ability to modify:

• publishConfig (although they can now also directly modify the main, exports, etc fields)
• version (can be handy to handle the versioning outside of Yarn itself)
• authors (for example to automatically generate the authors list from the git commit)

As well as all the other fields from package.json. They can even add new ones, for example to attach a commit link to the package.json during builds.

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.