fix: store forked derived values

[paoloricciuti]

Closes #17206

For some reason, despite a situation like this being the input for the second commit in #17163 we still ended up ignoring it.

This fixes it, however it introduces a new Map which I'm not too happy with...we could maybe do something with the batch itself?

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Latest commit: 5f05884

The changes in this PR will be included in the next version bump.

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Playground

pnpm add https://pkg.pr.new/svelte@17212

[sillvva]

This fixes #17206

Before: (hovering link causes dev console error)
https://stackblitz.com/edit/sveltejs-kit-template-default-j4cmhsez?file=package.json,src%2Froutes%2F%2Bpage.svelte,src%2Froutes%2Ftest%2F%2Bpage.svelte

After: (no error, link works as expected)
https://stackblitz.com/edit/sveltejs-kit-template-default-ypaqodke?file=package.json,src%2Froutes%2F%2Bpage.svelte,src%2Froutes%2Ftest%2F%2Bpage.svelte