(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.
This last Friday @joshcooper commented that he didn't want to see an explicit warning here (if I understood correctly.) If we're backslashifying forward slashes do we need to warn here?