parser/files: use a cheaper test for "is this path absolute?"

The parser used path == File.expand_path(path) as a test for the path being
absolute - something that wasn't entirely accurate, and that also wasn't
terrible fast. It also generated substantial memory and object allocation
traffic in the VM.

Replacing that with our util absolute_path? tool is faster and more efficient.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>