Annotate assembly references from NuGet packages with ResolveFrom metadata

@kzu commented on [Mon, 19 Sep 2016 18:37:30 GMT](https://github.com/NuGet/Home/issues/3468)
`@(ReferencePath)` from regular assembly references contain metadata like `ResolvedFrom={TargetFrameworkDirectory}` or `ResolvedFrom=ImplicitlyExpandDesignTimeFacades`. It would be nice if the references added by [ResolveNuGetPackageAssets](https://github.com/NuGet/NuGet.BuildTasks/blob/dev/src/Microsoft.NuGet.Build.Tasks/ResolveNuGetPackageAssets.cs#L311) annotated the reference as resolved by NuGet (maybe `ResolvedFrom=Package`?).

Currently, because this metadata isn't provided, they end up having `ResolvedFrom={RawFileName}`.

This would allow other tooling/targets to tell apart these references from others. (yes, I know the presence of the `NuGetPackageId` metadata can be used for the same, but for consistency with the way references are managed elsewhere, it would still be useful).