(PUP-2711) Make manifest-directory load recursively when future parser is on

This makes recursive loading of manifests available when the future parser is on. All .pp files in the manifestdir and its subdirectories will be loaded in alphabetical depth first order (a directory's entries are processed alphabetically, if it is a directory, it is processed before the remaining files in the current directory).
I just tried this out on windows. Somehow it works. Based on some error output I got at one point there is something that is translating windows path separators into unix path separators.