Use explicit extensions for imports within src

[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]

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 @sheetalkamat, @mjbvz, @zkat, and @joj for you. Feel free to loop in other consumers/maintainers if necessary.

[andrewbranch]

Something weird with the unused baselines check?

[jakebailey]

Weird, will take a look later.

[jakebailey]

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

[andrewbranch]

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

[jakebailey]

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

[andrewbranch]

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

[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.