jsii: also erase base types that exported from their module, but the module is not

[rix0rrr]

Classes that are exported from their module, but the module is not exported from the package should be erased as well.

Currently, they are not erased, and then the validation throws an error.

Marking them as @internal does not suffice, because then other TypeScript libraries can no longer use the package (since subclasses will still be declared as extendsing the hidden class, that is not written to a .d.ts file).

[github-actions]

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