OK, es könnte auch so gehen:
lib.contentElement {
templateRootPaths {
200 = EXT:your_extension_key/Resources/Private/Templates/
}
partialRootPaths {
200 = EXT:your_extension_key/Resources/Private/Partials/
}
layoutRootPaths {
200 = EXT:your_extension_key/Resources/Private/Layouts/
}
}