NuGet-based provider and CDN

[kzu]

For source-only nuget packages with (say) a single (or few) source file, it might be nice to be able to reference them directly just like libman provides for other package sources.

This would require defining a protocol (maybe augmenting the existing json-based one) and either supporting it built-in in nuget.org or allowing arbitrary sources (my preference, since that would also allow non-public feeds too).