[Bug]: Sourcemap is not generated for entry file which only do reexport in bundleless mode

Version

System:
    OS: macOS 14.6.1
    CPU: (10) arm64 Apple M1 Max
    Memory: 371.86 MB / 64.00 GB
    Shell: 5.9 - /bin/zsh
  Browsers:
    Chrome: 135.0.7049.96
    Safari: 17.6
  npmPackages:
    @rslib/core: ^0.6.5 => 0.6.5

Details

In bundleless mode, if we enable sourcemap generation, the source file which contain only reexport statement do not generate source map file.

// foo.ts
export const foo = "foo";

// index.ts
export { foo } from "./foo";

Reproduce link

https://github.com/Timeless0911/rslib-reexport-sourcemap

Reproduce Steps

1. pnpm install
2. pnpm build
3. check dist/bundleless folder that the index.js.map is missing which exists in dist/bundle folder.

The output result is as below:

dist/
├── bundle
│   ├── index.js
│   └── index.js.map
└── bundleless
    ├── foo.js
    ├── foo.js.map
    └── index.js

Start with the linked reproduction repository and run pnpm install followed by pnpm build. Compare the bundleless and bundle outputs in dist, focusing on why index.js lacks index.js.map when the entry file only reexports from foo.ts; done means the bundleless output includes the expected source map.