Interface cannot be hidden, but base class can

### Describe the feature

We have the feature where an exported class can `extend` a base class that's not exported, and jsii will take care of that properly.

The same feature does not extend to interfaces. If I `implement` a non-exported interface, I get errors.

### Use Case

I want a private interface for use by the CDK framework, mostly, and I'm not sure I want to expose it to customers.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

-

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

-

No files or tests are named. Start by tracing how jsii handles exported classes extending non-exported bases, then compare that behavior with interfaces implemented by exported classes; done means a non-exported interface can be implemented without errors while remaining hidden from generated customer APIs.

Written by the indexing model from the issue text.