(#8268) Require windows drive letters in absolute file paths

When testing whether a file path is absolute, the regexp was only
handling POSIX style file paths. This commit requires Windows
style file paths to start with a drive letter. A future commit
will refacter the various places we do path validation to
support both Windows drive letters and UNC paths.

Reviewed-by: Jacob Helwig <jacob@puppetlabs.com>
(cherry picked from commit 45ae5b4a9ced26dfcd3e324391f9a26cb02bf93d)