Refactor useage of Puppet::Util::Filetype

without this commit a file could not be created by the Wlxml based types.
This is troublesome since the prefetching will be conducted before
a potential file resource could at least ensure an empty file.

This commit lets the Puppet::Util::Filetype class work unimpeded to
create a new file if one didn't exits. Now the only prefetch failure
that should reasonably exist are from poorly formatted XML (a force
feature coming later), and bad filesystem permissions.