(#6820) Fix File class lookup in the file type for Ruby 1.9

Was getting the following error when we simply called File.dirname

err: Got an uncaught exception of type NoMethodError: undefined method
`dirname' for Puppet::Type::File:Class

The constant lookup has changed in Ruby 1.9 to look at local scopes
first, so we need to make it clearer that File is the Ruby File class
and not something scoped in Puppet.

Reviewed-by: Jacob Helwig <jacob@puppetlabs.com>