3.0.0-alpha.8 - extension addition causes failure in presence of exports in package.json

[benmccann]

Describe the bug

Vite adds an extension for deep imports:
https://github.com/vitejs/vite/pull/8348/files#diff-9b81bb364c02eab9494a7d27a5effc400cacbffd3b8f349c192f890c37bfc83fR639

vite-plugin-svelte resolves svelte as svelte/ssr when ssr: true:
https://github.com/sveltejs/vite-plugin-svelte/blob/0ee99f8ff7a18e7754f7bba373f0dce5817a12a0/packages/vite-plugin-svelte/src/index.ts#L142

Vite sees that the resolved path does not equal the original path and then adds .mjs. This file is not exposed in the exports and so it fails.

We should probably skip adding an extension when exports is present.

Reproduction

https://github.com/sveltejs/kit/tree/vite-3 - need to bump from alpha.6 to alpha.8 and then run pnpm build

System Info

3.0.0-alpha.8

Used Package Manager

pnpm

Logs

> Package subpath './ssr.mjs' is not defined by "exports" in packages/kit/node_modules/svelte/package.json imported from packages/kit/test/prerendering/options/.svelte-kit/output/server/index.js

Validations

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