(PDK-1045) Send validation targets as relative file paths

[bmjen]

Update the validator to parse targets and return results as file paths
relative to the module root. This is to cut down on the character length
of the command string, since the full path of the module directory is
unnecessary and long module_root paths may end up hitting the maximum
character limit on Windows.

[coveralls]

Coverage increased (+0.008%) to 92.799% when pulling c443c84 on bmjen:pdk_1045 into 4a73b87 on puppetlabs:master.

[rodjek]

It would be good to get an acceptance test for this scenario going as I'm worried that even with this change, doing a validate on a module with say 3000 files will still cause an issue on Windows.