Bug: Type aliases referenced inside index signatures results in an invalid .d.ts file

Hi! I've run into a bit of a bug with the bundling of .d.ts files. This was brought to my attention in an issue [here](https://github.com/airjp73/remix-validated-form/issues/227).

## Description

Bundling the following code outputs an invalid .d.ts file.

Input:
```ts
type Foo = string;

export type TestType = {
[key: `__${Foo}`]: any;
};
```

Output:
```ts
type TestType = {
[key: `__${Foo}`]: any;
};

export { TestType };
```

The output is missing the `Foo` type.

If you were to add `export type Bar = Foo` to the input, that would fix it and correctly include the `Foo` type in the .d.ts file.

## Reproduction repo

https://github.com/airjp73/index-type-error

Start with the linked reproduction repository and compare the generated declaration output for the provided index-signature example. Trace the declaration bundling entry point that omits the non-exported Foo alias, then confirm the fix produces a valid .d.ts file containing Foo without requiring an additional exported alias.

Written by the indexing model from the issue text.