Unraveled recursive call based planning transformation
* eliminated all custom flow control through usage of correctly defined templates called via `apply-templates`
** parameters are passed to the next recursion through a custom `level` node tree
*** this tree may be constructed using `construct_level`
* this approach is more flexible and more in line with how XSLT should be used
** it is now a more direct transformation between the source directory tree and the planned task structure
* analogously to the directory listing which was refactored in `26c0da`