In NuGet source, consider downloading licenseUrl even if license already known from file or expression

[zarenner]

Currently, we only download the nuspec's <licenseUrl> if we weren't able to determine the license in another way. That is, if we matched on a license file, <license type="expression">, or well-known <licenseUrl>, we don't download.

It's not clear that this is reasonable - for instance, what if the license text at <licenseUrl> doesn't match the same license as <license> or a LICENSE.txt? We should report other in those cases, but today we would report the license expression and/or file.

Like #269, this may require changes to licensee to better indicate if <license> and <licenseUrl> aren't guaranteed to match.