[api-extractor] Can not rollup DTS importing from index file

[Timeless0911]

Summary

When using dtsRollup feature, I found that it is unable to bundle types from importing from a index file but not write the full path.

Repro steps

https://github.com/Timeless0911/api-extractor-issue-index-path

1. pnpm install
2. pnpm build

Expected result:

see https://github.com/Timeless0911/api-extractor-issue-index-path/blob/main/dist-expected/index.d.ts

Actual result:

see https://github.com/Timeless0911/api-extractor-issue-index-path/blob/main/dist-actual/index.d.ts

Details

type-fest is a npm package which has a collection of essential TypeScript types that has below file structure:

.
├── index.d.ts
├── package.json
└── source
    ├── ...
    ├── delimiter-case.d.ts
    ├── internal
    │   ├── ...
    │   └── index.d.ts
    └── ...

This repro demo import KebabCase from type-fest

// src/index.ts
import type { KebabCase } from "type-fest";

export function kebabCase<T extends string>(str: T): KebabCase<T> {
  return str as KebabCase<T>;
}

The KebabCase type is from source/kebab-case.d.ts which have an import from ./delimiter-case

// source/kebab-case.d.ts
import type {DelimiterCase} from './delimiter-case';

// ....

export type KebabCase<Value> = DelimiterCase<Value, '-'>;

In source/delimiter-case.d.ts file, it import some types from ./internal which should be resolved to ./internal/index.d.ts

// source/delimiter-case.d.ts
import type {UpperCaseCharacters, WordSeparators} from './internal';

The actual result keeps these import from ./internal

import type { UpperCaseCharacters } from './internal';
import type { WordSeparators } from './internal';

I modify the source/delimiter-case.d.ts file to write full path of the importer like below, it works as expected.

- import type {UpperCaseCharacters, WordSeparators} from './internal';
+ import type {UpperCaseCharacters, WordSeparators} from './internal/index';

Since both Node.js and Typescript support index file resolution, I think ./internal should be resolved correctly when rolluping DTS.

Standard questions

Please answer these questions to help us investigate your issue more quickly:

Question	Answer
@microsoft/api-extractor version?	7.47.11
Operating system?	Mac
API Extractor scenario?	rollups (.d.ts)
Would you consider contributing a PR?	No
TypeScript compiler version?	5.4.2
Node.js version (node -v)?	22.10.0