APPVEYOR docs build doesn't apply memberpage plugin

[smaillet]

The memberpage plugin isn't being applied in the appveyor builds, or potentially on some local machine builds depending on the version of the dotnet SDK on the system.

This appears to be due to a change in the dotnet SDK between 2.1.202 and 2.1.302 where dotnet\sdk\2.1.302\Sdks\Microsoft.NET.Sdk\targets\Microsoft.PackageDependencyResolution.targets no longer produces the PackageDefinitions items that the Llv.Net.Docfx project custom GetDocfxPackagePaths depended on to locate the memberpage and MSDN xref nuget packages. Either the GetDocfxPackagePaths needs to be updated to trigger the target that will produce those MSBuildItems or a new mechanism for reliably locating those needs to be found.