(PUP-398) Adding warning to windows package provider source validation and test

Package installation/upgrade in Windows will probably fail if the source attribute path contains /

Not sure of all the ins and outs here, but I think warning is sufficient - it the path ends up really not existing to msiexec etc then the resource will fail, and the warning will give a hint as to the cause.  The warning should also promote best practice with windows paths.
I am easy either way, but a warn will only appear if we backslashify. The reason I only warned originally was in case forwardtobackslashification wasn't always required and might mess with the user in some way.  I left it in just so there was a hint to the user that /s are a bad idea and as a marker for unseen situations where maybe this breaks a thing.  Am happy to take it out if it just causes cruft.