`experimentalDts` generates broken type declarations.

# Problem

When using `experimentalDts` in a with components in a subfolder the generated output points to files that don't exist.

## Version info
tsup 8.0.1
@microsoft/api-extractor 7.38.3

## Example

```js
// input files
./src
components/
ComponentA/
index.tsx

// tsup config
export default defineConfig({
target: 'esnext',
platform: 'neutral',
format: 'esm',
experimentalDts: true,
entry: [ "./src/components/ComponentA.tsx" ]
});

// output files (Note how the components/ folder does not exist in dist)
./dist
ComponentA/
index.tsx
_tsup-dts-rollup.d.ts
```

Now when looking into the `dist/_tsup-dts-rollup.d.ts` file it contains references to `./components/ComponentA` resulting in broken typings.

## Proposed fix

The generated paths in `dist/_tsup-dts-rollup.d.ts` somehow reference nonexisting files when the entry code is in a subdirectory, so the fix would probably have to check the actual location of the output file and reference that instead.

## Other feedback

@ocavue
I originally tried out the `experimentalDts` option because I am building a number of entries (20+) and the dts build is very slow (~30 seconds). With the new experimentalDts option this time went down to ~7 seconds. Thanks for that great speed improvement. So it would be very nice to use if the generated paths were correct.