Include file extension in generated import specifiers

[developit]

What:

This outputs imports like import { pure } from "./pure.mjs"

Why:

This change should fix vitest-dev/vitest#1375.

ES Module specifiers are URLs with no implied extension or index file resolution. Tools like Vite relax this to allow CommonJS-style ./index.js resolution, but it's better to avoid relying on this since it's unspecified behavior.

How:

This uses the --out-file-extension option that was added to @babel/cli in version 7.8.0. It allows specifying a filename extension to use for generated import specifiers.

Checklist:

• N/A Documentation added
• N/A Tests
• N/A Typescript definitions updated
• Ready to be merged

[JoviDeCroock]

I have quickly tried to do this in my example linked in the issue which results in the same issue.

• I removed all .js files
• Replaced the imports to have .mjs extensions
• Replaced it in the pkg.json import maps

Which still results in them being unfound for me 😅 doing the exact same thing with .js works perfectly but in turn seems to break Node (unless we add type module but then the cjs folder breaks)

[JoviDeCroock]

Closing as superseded by #60