Stubs for internal nested classes get created

[lars-reimann]

Describe the bug

Stubs for internal nested classes are created:

class ImageList() {
    @PythonName("_FromFileThreadPackage")
    class FromFileThreadPackage(
        @PythonName("im_files") imFiles: List<String>,
        @PythonName("im_channel") imChannel: Int,
        @PythonName("to_channel") toChannel: Int,
        @PythonName("im_width") imWidth: Int,
        @PythonName("im_height") imHeight: Int,
        tensor: Tensor,
        @PythonName("start_index") startIndex: Int
    )

    @PythonName("_FromImageThread")
    class FromImageThread(
        packages: List<_FromFileThreadPackage>
    ) sub Thread
}

To Reproduce

Create stubs for the safe-ds library, check the created stubs in data\image\containers\ImageList.sdsstub.

Expected behavior

No stubs for any internal API elements.

Screenshots (optional)

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Additional Context (optional)

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