Bug when rewriting relative import in un-optimized node_modules

[csr632]

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Describe the bug

In this line, the ctx.path may be a direcory.
https://github.com/vuejs/vite/blob/6683bb8fb819c6f4935b40f25c2a377037e5ec7d/src/node/server/serverPluginModuleRewrite.ts#L129

But when you resolve relative import, you treat it as a file path.
https://github.com/vuejs/vite/blob/49a44b648f263ff058f730913ea1ee6c62e3cd2d/src/node/utils/pathUtils.ts#L16

For example, if /dir/index.js have import './utils', and browser request for /dir, vite will try to find utils from /utils instead of /dir/utils

[yyx990803]

importer is always the resolved full path with filename because we pre-resolve the imports (i.e. the file with import './dir' will already have been rewritten to import './dir/index.js').

Please make sure to only open issues when you reproduce it.

[csr632]

@yyx990803

Here is a minimal reproduce:
https://github.com/csr632/test-vite/tree/wrong-resolve

It happends when I config optimizeDeps.auto = false.
This bug only happends when rewriting import in un-optimized node_modules.