(PUP-121) Make include, require, and contain enforce non relative names

This makes include, require, and contain functions transform
relative names to absolute names before doing any lookup.

The transformation is controlled by a metho on scope that is
always called, and that performs the transformation when Puppet[:parser]

future. This design was selected to ensure that there is one place to

later change (other flags, or in Puppet 4 when the future will
be something else).