fix: preserve TypeScript imports for ESM builds

[Jaksenc]

Summary

• preserve TypeScript ES imports when ncc has selected ESM output but tsconfig.json asks TypeScript to emit a known CommonJS-style module format
• leave user TypeScript module settings alone for non-ESM builds and other module modes
• add a regression test for an ESM package that exposes only an import condition

Fixes #1311.

Why

@actions/core@3.0.0 is ESM-only: its package root has type: "module" and an exports["."].import entry, but no require condition. In a TypeScript project with type: "module" and compilerOptions.module set to CommonJS, ncc correctly selects ESM output, but ts-loader lowers the entry import to require(). Webpack then resolves the dependency through CommonJS conditions and fails with Package path . is not exported.

The fixture sets esm: true explicitly so the test focuses on ncc's ESM output path while keeping the TypeScript module: "commonjs" setting that triggers the regression.

This patch keeps TypeScript imports intact only for that ESM-output/CommonJS-emit mismatch, so webpack can resolve import-only packages through their import condition without broadening the behavior for CJS ncc builds.

Research / Repro

• Confirmed @actions/core@3.0.0 exposes package root through import only via npm view @actions/core@3.0.0 version type exports --json.
• Confirmed TypeScript parses compilerOptions.module: "CommonJS" to ModuleKind.CommonJS, which is the known CommonJS-style emit mode covered here.
• Ran the new repro against origin/main in a detached worktree: it fails with Package path . is not exported for an import-only package.
• Ran the same repro against this branch: it passes and the bundled code contains the import-only package value.
• Checked open PR overlap for github/core package fails build because of exports map #1311; only this draft PR currently targets it.

Testing

• ./node_modules/.bin/tsc --noEmit -p test/unit/ts-esm-import-condition/tsconfig.json
• node --expose-gc --max_old_space_size=4096 node_modules/jest/bin/jest.js test/unit.test.js --runInBand --testNamePattern ts-esm-import-condition
• node --expose-gc --max_old_space_size=4096 node_modules/jest/bin/jest.js test/unit.test.js --runInBand --coverage --globals '{"coverage":true}' --testNamePattern ts-esm-import-condition

[styfle]

Where are these integers defined? Lets link to the docs so its clear.

[Jaksenc]

Ah yeah, that’s fair. I added a link to the TypeScript enum here and renamed the helper so it’s clearer what those numbers are doing.

[styfle]

Why is this a temp directory? Looks like you're building a test fixture so lets just create a new test fixture and commit it since this doesn't need any dynamic behavior

[Jaksenc]

Youre right that makes more sense, just changed it. Thanks!!

[styfle]

This is super strange behavior since you have module=commonjs in tsconfig.json here but type=module in package.json

Does this code even compile and run using tsc + node?

I dont understand why ncc should support this 🤔

[Jaksenc]

No you’re so right, the fixture makes the case look a bit weird.

I was trying to test ncc’s ESM output path instaed of saying that this setup should be run directly with tsc + Node. Lemme clean it up by moving the import-only package into node_modules and making esm: true explicit in the fixture options so the test is clearer.