account for server-emitted assets when prerendering

[Rich-Harris]

closes #9146. I couldn't quite get to the bottom of the specifics of that issue's reproduction, because some very strange things were happening that resulted in missing assets in `output/client. I think there's another bug somewhere, possibly in the bowels of Vite, but it eluded me.

Anyway, the prerenderer isn't taking account of server-emitted assets, and this PR fixes it.

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[dummdidumm]

chirp chirp