Cannot privately inherit a non-exported class from a different file

### Describe the bug

The issue is that:

```ts
class Private { }

export class Public extends Private { }
```

Is allowed, while:

```ts
// private.ts
export class Private { }

// index.ts
import { Private } from './private';
export class Public extends Private { }
```
Is not.

I've been told the workaround is to tag the private type with `@internal`, but the current error message looks like:

```
lib/gen/private/webaclgen.generated.ts:71:1 - error JSII9002: Unable to resolve type "@aws-cdk/aws-wafv2.WebAclGen". It may be @internal or not exported from the module's entry point (as configured in "package.json" as "main").

71 export class WebAclGen extends Resource implements IWebAcl {
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
72 public static fromWebAclAttributes(scope: Construct, id: string, attrs: WebAclAttributes): IWebAcl {
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
...
116 }
~~~
117 }
~
```

Which seems to imply that `@internal` is a Bad Thing and should be avoided.

### Expected Behavior

See above.

### Current Behavior

See above.

### Reproduction Steps

See above.

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### SDK version used

-

### Environment details (OS name and version, etc.)

-

Start by reproducing the two TypeScript inheritance examples and inspect the diagnostic reported at lib/gen/private/webaclgen.generated.ts:71, along with the package.json main entry point mentioned in the error. Trace how cross-file non-exported classes and @internal types are classified; done means the examples receive consistent, accurate handling and the diagnostic clearly explains the supported workaround.

Written by the indexing model from the issue text.