Revert "force empty_folders hook to run last by adding z_ prefix. This is a hack"

This reverts commit be739ea6d8c191b1eae42e793c70151b1103cba7.

We should do this cleanly instead.
Ensure that the empty_folders clean up hook runs last for --clean=all
by making sure it is last in the array of the detected hooks. This
is necessary because a directory might not become empty until all the
other hooks have removed unwanted files/directories.