Combine javadocs of subprojects: too many module declarations

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1

I use this to try to create an aggregate javavadoc of all subprojects, alongside the jar that combines all subproject jars (next up would be combined sources):

task javadocs(type: Javadoc) {
	source subprojects.collect { project ->
		project.sourceSets.main.allJava
	}
	println buildDir
	destinationDir = new File(buildDir, 'javadoc')
}

This task fails with the error too many module declarations found. Each of my subprojects has a separate module-info.java that now get listed. What would be a workaround here?