CORS check should not apply to +page|layout.server.js

[Rich-Harris]

Describe the bug

In #6550 we started simulating CORS errors for server-side fetches to avoid a discrepancy between server-side and client-side fetches.

More recently, in #7113, we added fetch to RequestEvent, meaning that you can use event.fetch in a +page.server.js or +layout.server.js. But the simulated CORS errors still apply, needlessly.

Reproduction

https://github.com/Rich-Harris/sveltekit-server-side-fetch-bug-repro

Logs

No response

System Info

System:
    OS: macOS 12.5.1
    CPU: (10) arm64 Apple M1 Max
    Memory: 74.27 MB / 32.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 16.17.0 - ~/.nvm/versions/node/v16.17.0/bin/node
    npm: 8.15.0 - ~/.nvm/versions/node/v16.17.0/bin/npm
  Browsers:
    Chrome: 107.0.5304.87
    Chrome Canary: 109.0.5388.0
    Firefox: 106.0.1
    Safari: 15.6.1
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.86 
    @sveltejs/kit: next => 1.0.0-next.511 
    svelte: ^3.44.0 => 3.52.0 
    vite: ^3.1.0 => 3.2.2

Severity

serious, but I can work around it

Additional Information

No response

[dummdidumm]

Didn't think I'd say this to you one day, but duplicate of #7441 😜