(#5477) Allow watch_file to watch non-existent files, especially site.pp

The watch_file mechanism would refuse to monitor paths to files that
didn't exist. This patch makes it possible to watch a file that hasn't
been created yet, so when it is created, you manifests will get
reparsed.

Backported this change to 2.6.x

Paired-With: Jacob Helwig <jacob@puppetlabs.com>