PackageDownload: support GeneratePathProperty

[KirillOsenkov]

If my understand is correct, PackageDownload items currently don't support GeneratePathProperty. This loses a lot of potential value prop because in the scenarios where you use PackageDownload you're highly likely to want to know a deterministic path to the package contents where it was laid out on disk.

Right now the only way we can see is rely on $(NuGetPackageRoot) which is problematic because I've seen users remap it to say E:\NuGet (without the trailing slash), wreaking all sorts of havoc.

[KirillOsenkov]

@nkolev92 @sandyarmstrong

[sandyarmstrong]

Even with $(NuGetPackageRoot) you have to repeat a lot of information to get the full nupkg path. It would be nice to have GenerateNupkgPathProperty or something too. :-)

[nkolev92]

Dup of #8476.