(#8489) Consistently use File::PATH_SEPARATOR

Puppet uses both colon and File::PATH_SEPARATOR in various places, which
does not work on Windows, where File::PATH_SEPARATOR is a semi-colon. This
commit changes the code and tests to consistently use File::PATH_SEPARATOR.

Reviewed-by: Jacob Helwig <jacob@puppetlabs.com>
(cherry picked from commit 26ee468e8b963d63933d9a27a65d55510ff87618)