Use explicit extensions for imports within src

[Jake Bailey (jakebailey)]

This is extracted from #58419.

Doing this now would reduce churn, but I'm not 100% certain how we can avoid adding the "wrong" kind of imports in the future before "type": "module"? Lint rule!

[TypeScript Bot (typescript-bot)]

Thanks for the PR! It looks like you've changed the TSServer protocol in some way. Please ensure that any changes here don't break consumers of the current TSServer API. For some extra review, we'll ping Sheetal Nandi (@sheetalkamat), Matt Bierner (@mjbvz), Kat Marchán (@zkat), and Joaquin Jares (@joj) for you. Feel free to loop in other consumers/maintainers if necessary.

[Andrew Branch (andrewbranch)]

Something weird with the unused baselines check?

[Jake Bailey (jakebailey)]

Weird, will take a look later.

[Jake Bailey (jakebailey)]

Andrew Branch (@andrewbranch) Is there a way to force this to always be required, even though we're currently being treated as CommonJS?

[Andrew Branch (andrewbranch)]

Not in tsc. You can set "importModuleSpecifierEnding": "js" in VS Code preferences for auto imports, and otherwise use a lint rule to enforce.

[Jake Bailey (jakebailey)]

I'll whip up a lint rule and set that, thanks!

[Andrew Branch (andrewbranch)]

I know eslint-plugin-import has a rule for that; not sure about others.

[Jake Bailey (jakebailey)]

Sanity checked, and reverting all changes in src, running hereby lint --fix and dprint nets the right result, missing only the manual changes for using export * for the unit test reexport file (no runtime difference, but it makes the imports checked), and then two unit test loading hacks that are in place due to our goofy test runner.