`adapter-cloudflare` seems to undo all code splitting, and worse

Describe the bug

I'm using cloudflare-adapter to deploy to Cloudflare Pages. My _worker.js is growing to ridiculous sizes, perhaps most relevantly, it's larger than my app with all its chunks!

$ du -sh --apparent-size .svelte-kit/output/ .svelte-kit/cloudflare/
1.2M	.svelte-kit/output/
3.4M	.svelte-kit/cloudflare/

My biggest dependency is mapboxgl, and I see it minified in .svelte-kit/cloudflare/**/_app/chunks/vendor-*.js. But, I also see it embedded in .svelte-kit/cloudflare/_worker.js. Why is it both split and not split?

Reproduction

TBD

Logs

$ npm run build
[ everything here is under 44 KiB ] 
.svelte-kit/output/client/compare/_app/chunks/vendor-6f46024b.js                                        901.61 KiB / gzip: 246.75 KiB

(!) Some chunks are larger than 500 KiB after minification. Consider:
- Using dynamic import() to code-split the application
- Use build.rollupOptions.output.manualChunks to improve chunking: https://rollupjs.org/guide/en/#outputmanualchunks
- Adjust chunk size limit for this warning via build.chunkSizeWarningLimit.
vite v2.6.13 building SSR bundle for production...
✓ 77 modules transformed.
.svelte-kit/output/server/app.js   106.58 KiB

Run npm run preview to preview your production build locally.

> Using @sveltejs/adapter-cloudflare
  ✔ done

System Info

System:
    OS: Linux 5.14 NixOS 21.11 (Porcupine) 21.11 (Porcupine)
    CPU: (8) x64 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
    Memory: 36.93 GB / 62.61 GB
    Container: Yes
    Shell: 5.1.8 - /nix/store/xdxv22yb7qnjfyf997r9fx804761z14c-bash-interactive-5.1-p8/bin/bash
  Binaries:
    Node: 16.13.0 - /nix/store/26lslfmvwyhspppbinznh111fyaxx62z-nodejs-16.13.0/bin/node
    npm: 8.1.0 - /nix/store/26lslfmvwyhspppbinznh111fyaxx62z-nodejs-16.13.0/bin/npm
  Browsers:
    Firefox: 94.0.2
  npmPackages:
    @sveltejs/adapter-cloudflare: ^1.0.0-next.2 => 1.0.0-next.2
    @sveltejs/kit: next => 1.0.0-next.191
    svelte: ^3.34.0 => 3.44.0

Severity

blocking an upgrade

Additional Information

This is severe because Cloudflare Workers _worker.js limit is only 1.5 MB, and importing mapboxgl without splitting uses a huge part of that.

Start by reproducing the issue with the reported npm run build flow and inspect .svelte-kit/cloudflare/_worker.js alongside .svelte-kit/cloudflare/**/_app/chunks/vendor-*.js. Compare the generated worker with .svelte-kit/output/server/app.js to determine why dependencies appear in both places. Done means the Cloudflare worker no longer unnecessarily duplicates code-split dependencies and remains within the platform size limit.