chore: use peer dependencies rather than dev dependencies

[Rich-Harris]

Right now it's rather difficult to use Svelte 5 alongside a linked @sveltejs/kit, because Kit imports vite and @sveltejs/vite-plugin-svelte relative to itself, even though these are peer dependencies.

This means that components will be compiled by Svelte 4 (as that's what the @sveltejs/vite-plugin-svelte inside the linked @sveltejs/kit depends on), but then try to use the Svelte 5 runtime.

This is already a nuisance but it will become much more of one as Svelte 5 gets closer to release and we're trying to make sure everything works in both versions.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 39e4fe5

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[maximdeclercq]

This PR makes use of import-meta-resolve, which is incompatible with Yarn PnP, resulting in the following error:

error when starting dev server:
Error: Cannot find package '@sveltejs/vite-plugin-svelte' imported from /work/my-app/dummy.js
    at __node_internal_ (file:///home/mdc/.yarn/berry/cache/import-meta-resolve-npm-4.0.0-13a6ef4fd3-10c0.zip/node_modules/import-meta-resolve/lib/errors.js:431:11)
    at new NodeError (file:///home/mdc/.yarn/berry/cache/import-meta-resolve-npm-4.0.0-13a6ef4fd3-10c0.zip/node_modules/import-meta-resolve/lib/errors.js:374:5)
    at packageResolve (file:///home/mdc/.yarn/berry/cache/import-meta-resolve-npm-4.0.0-13a6ef4fd3-10c0.zip/node_modules/import-meta-resolve/lib/resolve.js:1038:9)
    at moduleResolve (file:///home/mdc/.yarn/berry/cache/import-meta-resolve-npm-4.0.0-13a6ef4fd3-10c0.zip/node_modules/import-meta-resolve/lib/resolve.js:1101:20)
    at defaultResolve (file:///home/mdc/.yarn/berry/cache/import-meta-resolve-npm-4.0.0-13a6ef4fd3-10c0.zip/node_modules/import-meta-resolve/lib/resolve.js:1266:15)
    at Module.resolve (file:///home/mdc/.yarn/berry/cache/import-meta-resolve-npm-4.0.0-13a6ef4fd3-10c0.zip/node_modules/import-meta-resolve/index.js:32:12)
    at resolve_peer_dependency (file:///work/my-app/.yarn/unplugged/@sveltejs-kit-virtual-ff81c722c8/node_modules/@sveltejs/kit/src/exports/vite/index.js:132:29)
    at sveltekit (file:///work/my-app/.yarn/unplugged/@sveltejs-kit-virtual-ff81c722c8/node_modules/@sveltejs/kit/src/exports/vite/index.js:167:27)
    at async Promise.all (index 0)
    at async asyncFlatten (file:///work/my-app/.yarn/__virtual__/vite-virtual-ccd3afe941/4/.yarn/berry/cache/vite-npm-5.0.11-d5457a8b86-10c0.zip/node_modules/vite/dist/node/chunks/dep-V3BH7oO1.js:12894:16)
    at async resolveConfig (file:///work/my-app/.yarn/__virtual__/vite-virtual-ccd3afe941/4/.yarn/berry/cache/vite-npm-5.0.11-d5457a8b86-10c0.zip/node_modules/vite/dist/node/chunks/dep-V3BH7oO1.js:67863:29)
    at async _createServer (file:///work/my-app/.yarn/__virtual__/vite-virtual-ccd3afe941/4/.yarn/berry/cache/vite-npm-5.0.11-d5457a8b86-10c0.zip/node_modules/vite/dist/node/chunks/dep-V3BH7oO1.js:60327:20)
    at async CAC.<anonymous> (file:///work/my-app/.yarn/__virtual__/vite-virtual-ccd3afe941/4/.yarn/berry/cache/vite-npm-5.0.11-d5457a8b86-10c0.zip/node_modules/vite/dist/node/cli.js:764:24)

[Rich-Harris]

Yarn PnP is broken, and we do not support it. Use npm or pnpm instead.

[maximdeclercq]

Yarn PnP with Yarn 4 works on SvelteKit 2.0.7, but not on SvelteKit 2.0.8.
I have made an issue here #11578.

[clemyan]

You can restore compatibility if you fallback to just import-ing if cwd-relative import fails. Given that this was the original behavior before this change and this change is aimed at maintainers/contributors to SvelteKit I don't think that's an unreasonable ask.

EDIT: I don't use Svelte in my day-to-day so I don't know if this is feasible/common, but if there are wrapper packages around SvelteKit, looks like this change would break the setup of a developer linking to that wrapper.

And, FWIW, you wouldn't need this change at all if you were using PnP -- it can correctly resolve peer dependencies across portals (Yarn's equivalent to npm link)