(#12101) Make Puppet::Util::absolute_path? usable in autoloader

This makes Puppet::Util::absolute_path? check File::ALT_SEPARATOR to determine
path behavior of the local system instead of Puppet.features, which required
having an Autoload instance already. File::ALT_SEPARATOR is only defined on
systems with Windows path behavior, and the Ruby standard library uses this
frequently to distinguish systems.