Import Path Issues with `bundle: false` and Suggestion for Built-in Fixes

Hi,

First, thank you for the fantastic work on `tsup`!

I've encountered some import path issues when using `tsup` with `bundle: false`. Specifically, the problems are:

1. **Missing File Extensions**: When `bundle: false` is set, the output files sometimes have import statements without explicit file extensions (like `.js`, `.mjs`, or `.cjs`). This can cause runtime errors in environments that require explicit file extensions.

2. **Directory Imports**: Importing from directories without specifying the `index` file (e.g., `import { something } from './utils';`) can lead to module resolution issues because some environments don't automatically resolve to `index` files.

3. **Unresolved Path Aliases**: Path aliases defined in `tsconfig.json` aren't correctly resolved in the output when not bundling, resulting in broken imports.

To address these issues, I created a package called [`esbuild-fix-imports-plugin`](https://www.npmjs.com/package/esbuild-fix-imports-plugin) that combines three ESBuild plugins to modify the output files:

- **`fixAliasPlugin`**: Resolves path aliases from `tsconfig.json` to relative paths.
- **`fixFolderImportsPlugin`**: Converts directory imports to explicit paths pointing to `index` files.
- **`fixExtensionsPlugin`**: Appends correct file extensions to relative import paths.

**Suggestion**:

Would it be possible to integrate similar fixes directly into `tsup` when `bundle: false` is used? This could greatly enhance the developer experience by:

- Automatically appending the correct file extensions to imports in the output.
- Resolving path aliases defined in `tsconfig.json`.
- Adjusting directory imports to explicitly reference `index` files.

**Example of the Issues**:

Given a source file with:

```typescript
import { myFunction } from './utils'; // Importing using folder path
import { myFunction } from './utils/myFunction'; // Importing using file path
import { myAliasFunction } from '@alias/utils/myFunction'; // Importing using alias
```

After building with `tsup` and `bundle: false`, the output might contain:

```javascript
const { myFunction } = require('./utils');
const { myFunction } = require('./utils/myFunction');
const { myAliasFunction } = require('@alias/utils/myFunction');
```

This can cause runtime errors due to unresolved imports.

**Proposed Solution**:

Integrate logic similar to the plugins in `esbuild-fix-imports-plugin` to process the output files when `bundle: false` is set. This could be an opt-in feature or enabled by default when not bundling.

**Alternative**:

If direct integration isn't feasible, perhaps the documentation could mention this issue and recommend using `esbuild-fix-imports-plugin` or similar solutions.

**Additional Context**:

Here's how I currently use the plugin with `tsup`:

```typescript
import { defineConfig } from 'tsup';
import { fixImportsPlugin } from 'esbuild-fix-imports-plugin';

export default defineConfig({
// Other configurations
bundle: false,
esbuildPlugins: [fixImportsPlugin()],
// ...
});
```

**References**:

- [esbuild-fix-imports-plugin on npm](https://www.npmjs.com/package/esbuild-fix-imports-plugin)
- [esbuild-fix-imports-plugin GitHub repo](https://github.com/aymericzip/esbuild-fix-imports-plugin)

Thank you for considering this suggestion!

Best regards,

Start by reviewing the `defineConfig` options for `bundle: false` and `esbuildPlugins`, then compare their behavior with the referenced `esbuild-fix-imports-plugin`. Define coverage for missing extensions, directory imports, and `tsconfig.json` path aliases, and determine whether the behavior should be opt-in or documented as an alternative. Done means the chosen fixes work reliably or the limitation is clearly documented.

Written by the indexing model from the issue text.