reorganise codebase a bit

[Rich-Harris]

This started out with me working on #4051 and immediately wanting to move packages/kit/src/hooks.js to packages/kit/src/hooks/sequence.js so that we could have a sequence.spec.js alongside it, but not wanting to do that because it would make src even more raggedy than it currently is.

As of this PR, the contents of src look like this...

src/
├ core/     # the dev/build logic
├ exports/  # stuff exposed as the `@sveltejs/kit` package
├ runtime/  # code that runs when people use a SvelteKit app
├ utils/    # self-explanatory
└ cli.js    # self-explanatory

...which I think makes more sense than src/vite, src/node etc. There's definitely more that could be done here (the runtime directory in particular has become a bit chaotic), but sorting out the top level feels like a good start, and would make me feel calmer when I'm working in the codebase.

No changeset because this change should be invisible to users. I used git mv to preserve history and minimise the chance of merge conflicts.

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[benmccann]

This seems pretty good overall.

The contributing guide would need to be updated.

It feels a little weird to me that core doesn't contain the most important code paths. Should it be called build or something?

(the directory tree in the description gave me a chuckle. I think someone's been typing src/routes a few too many times 😄)

[dummdidumm]

I agree with core being somewhat clunky right now - for example, sync is much closer to runtime yet it's in a separate tree. But we can do this in a separate PR.

[dummdidumm]

Test failures seem unrelated/due to flakyness. Will do one rerun to be sure.

Edit: All test failures except one (which looks like flakyness) are related to the $env thing - not sure why it fails this consistently now, it didn't do that before I believe.

[Rich-Harris]

The $env thing is a red herring. We should probably make it less noisy. It's actually #6306

[Rich-Harris]

The distinction between core and runtime is roughly this: runtime is the stuff that runs through Vite during dev and is included in the app once it's built, core is the stuff that runs in Node and only during dev/build time. It definitely wouldn't make sense for sync to be in runtime, for example.