Compiler error when using @PolymorphicEnumCodable macro on enums defined inside nested types

[victorhenrion]

Similar to #15, you can't use @PolymorphicEnumCodable inside a nested type: it generates an incorrect extension.

Reproduction:

import Foundation
import KarrotCodableKit

enum SomeEnum {
    @PolymorphicEnumCodable(identifierCodingKey: "type")
    enum ViewItem {
        case image(ImageViewItem)
        case text(TextViewItem)
    }
    
    @PolymorphicCodable(identifier: "IMAGE_VIEW_ITEM")
    struct ImageViewItem {
        let id: String
        let imageURL: URL
    }
    
    @PolymorphicCodable(identifier: "TEXT_VIEW_ITEM")
    struct TextViewItem {
        let id: String
        let title: String
        let description: String
    }
}