Fix double-mount bug

[Rich-Harris]

Stumbled upon a nasty bug while investigating #4864 — if you edit src/routes/a/[b].svelte, then any navigation from /a/1 to /a/2 will cause [b].svelte to be reloaded (with the HMR cache-busting timestamp), then unmounted and remounted.

I think the reason is that we have an import('/src/routes/[b].svelte') in the HTML and in the routes manifest; the latter import is cachebusted but the one in the HTML isn't.

This only occurs during dev.

• fixes Double component mount on $session change after component change and page refresh in the dev mode #4799
• fixes __layout.svelte may run twice after browser reload in dev #2130

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Latest commit: d402526

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[Rich-Harris]

The reason this is failing CI is that changing how the initial page components are loaded has surfaced an unrelated bug: vitePreload fails in svelte-kit preview if paths.assets is specified, because in preview we replace it with /_svelte_kit_assets in order to serve assets locally. Previously the bug was masked (at least in the tests) because the components were already loaded via import(...) and <link>, meaning the router didn't try to load them via vitePreload.

This will hopefully be fixed by vitejs/vite#7644