fix(ssr): (backport #18150) fix source map remapping with multiple sources

[hi-ogawa]

Description

Backporting #18150 to unblock debugger issues on Vitest vitest-dev/vitest#6526.
It doesn't need to be released asap but it would be great to have if there's a chance for next batch of v5 patch.

[bolt-new-by-stackblitz]

Run & review this pull request in StackBlitz Codeflow.

[sapphi-red]

/ecosystem-ci run

[vite-ecosystem-ci]

📝 Ran ecosystem CI on e47b220: Open

suite	result	latest scheduled
histoire	✅ success	❌ failure
remix	✅ success	❌ failure
sveltekit	❌ failure	✅ success
vike	✅ success	❌ failure
vuepress	❌ failure	✅ success

✅ analogjs, astro, ladle, laravel, marko, nuxt, previewjs, quasar, qwik, rakkas, redwoodjs, storybook, unocss, vite-environment-examples, vite-plugin-pwa, vite-plugin-react, vite-plugin-react-swc, vite-plugin-svelte, vite-plugin-vue, vite-setup-catalogue, vitepress, vitest

[hi-ogawa]

Vuepress fail says playwright installation issue https://github.com/vitejs/vite-ecosystem-ci/actions/runs/11098468509/job/30834087016#step:8:1566

Sveltekit fail is something I saw before in #18152 (comment), so probably flaky https://github.com/vitejs/vite-ecosystem-ci/actions/runs/11098468509/job/30834085391#step:8:2377

[sapphi-red]

/ecosystem-ci run sveltekit

[vite-ecosystem-ci]

📝 Ran ecosystem CI on e47b220: Open

✅ sveltekit

[sapphi-red]

/ecosystem-ci run vuepress

[vite-ecosystem-ci]

📝 Ran ecosystem CI on e47b220: Open

✅ vuepress

[sapphi-red]

Yup, everything passed 👍