'GeneratePathProperty' does not work on 'PackageDownload'

[sharwell]

The new <PackageDownload> elements do not support the GeneratePathProperty attribute, making it difficult to download a package whose contents will be used by an unrelated build task.

[nkolev92]

I'd be open to adding this eventually.
The thing is PackageDownload allows duplicate ids, so we'd need a different naming schema compared to PackageReference.

Note that because of the exactness of PackageDownload (specifically, only exact versions allowed), you can create the path fairly accurately as it will be:
$(NuGetPackageRoot)\$(PackageId)\$(PackageVersion)