use_plugin_extension.kt

class BundleReleaseFilesPlugin : Plugin<Project> {

    override fun apply(target: Project) {
        ...
        bundleReleaseFiles.configure {
            appVersion.set(extension.appVersion)
            rootProject.set(target.projectDir) // We get the rootProject from the "target" parameter
            outputDirectory.set(extension.outputDirectory)
        }
    }
}