chore: simplify/optimize source_ownership occurrence

[Rich-Harris]

was looking into whether we could use the new update_version mechanism to make it cheaper to answer 'was this source created inside the current reaction?', and noticed that we're currently doing a check inside a loop that should be outside, since the answer is always the same

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16328

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[elliott-with-the-longest-name-on-github]

I don't see any code inside the loop that would cause side effects so I think it's good...