Update MicrosoftPrivateIntellisensePackage and XmlDocFileRoot

[carlossanlop]

XmlDocFileRoot was defined in two places: packaging.props and docs.targets.
Moved it to a higher level to ensure it's only defined once, in Directory.Build.props. Made sure to define it after NuGetPackageRoot is defined.
Also made sure the folder it reads is net instead of netcoreapp, because the Docs build saves the .NET 5.0 intellisense files in the net folder, and the .NET 3.1 intellisense files in the netcoreapp folder.
Updated the nupkg to consume to the latest available version in the feed 20200602.3, which contains the Preview5 APIs.

I will make a similar change in the Preview6 branch today.
Thanks @safern for your help in figuring out what changes needed to be made.

[ghost]

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[carlossanlop]

Found a build error locally. Santi and I are investigating.

[safern]

It seems like the only shared location were we could define XmlDocFileRoot is in Versions.props -- because this is used in pkgproj and Build.proj restore.

[ViktorHofer]

For such shared properties we use Configurations.props.

[safern]

We can’t use Configurations.props. We tried it already.

The reason for that is that when we’re building a pkgproj; arcade is imported after Configurations.props is imported and NugetPackageRoot is not defined at the moment this property is defined.

We disable importing arcade from the root for anything under src/libraries. The only place I could think of was Versions.props where in both code paths (Build.proj restore and pkgproj build) arcade would already had defined NugetPackageRoot

[ViktorHofer]

Makes sense. We should enable the root Arcade import. The reason I had to disable that was because we relied on the old configuration system. I believe there aren't any blockers anymore.

[safern]

Cool. Can we do that as part of another change? I believe there was an issue for that, right? I just don’t want to complicate this change.

[ViktorHofer]

Sure