interface A extends B fails if B is exported from a private file

The following succeeds:

```ts
export interface A extends B { /* ... */ }

interface B { /* ... */ }
```

It will "hide" the interface B entirely from jsii.

But the following fails:

```ts
import { B } from './private/b';

export interface A extends B { /* ... */ }
```

With the error "can't find B, make sure it's exported".

Even though it's the exact same situation.

```
aws-cloudwatch/lib/alarm.ts:33:1 - error JSII9002: Unable to resolve type "aws-cdk-lib.CreateAlarmOptionsBase". It may be @internal or not exported from the module's entry point (as configured in "package.json" as "main").
```

Adding `@internal` to `B` does appease jsii, but this still shouldn't happen this way.

## Probable reason

I'm guessing that the code that checks whether or not the `B` type should be hidden or should be visible looks at the `export` keyword and nothing else, but it `B` *must* have `export` if it comes from a different file, even if that file is itself not exported on purpose.

Start by reproducing the two interface examples and inspect the type-export visibility logic that reports JSII9002, using aws-cloudwatch/lib/alarm.ts:33:1 as the failing example. Add a regression test showing that an exported interface extending a type exported from a private file is treated like one extending a local non-exported type; done means the imported case compiles without requiring @internal.

Written by the indexing model from the issue text.