Sealed Interfaces with named companion objects do not generate serializers

[Shengaero]

Describe the bug
Named companion objects of sealed interfaces do not auto-generate serializers when tagged with @Serializable.

Note that this only occurs when the companion object is named, if unnamed this error will not occur.

To Reproduce

import kotlinx.serialization.SerialName
import kotlinx.serialization.Serializable
import kotlinx.serialization.encodeToString
import kotlinx.serialization.json.Json

@Serializable
sealed interface MyInterface {
    companion object NamedCompanionObject {
        const val Example = "testing"
    }
}

@Serializable
@SerialName("FooBar")
data class FooBar(val value: String): MyInterface

fun main() {
    // error will be thrown here, reporting that MyInterface is not tagged with @Serializable
    val text = Json.encodeToString<MyInterface>(FooBar(MyInterface.Example))
    println(text)
}

Expected behavior
This should not throw an error, or at least I don't believe it should.
Nothing in the documentation I could find implies this is expected behavior.

Environment

• Kotlin version: 1.8.0
• Library version: 1.4.1
• Kotlin platforms: JVM
• Gradle version: 7.4.2
• IDE version: Unsure if related but IntelliJ IDEA 2022.2.3

[shanshin]

Related #1291, #1207