[Directory.Build.props] NuGet package not restored when defining StyleCop in Directory.Build.props

In Visual Studio 2019 Community (Version 16.7.0), I've defined Directory.Build.props as following:

```

$(SolutionDir)StyleCop.ruleset

```

Then I removed the packages folder, and tried to restore NuGet packages (right-click on solution and "Restore NuGet Packages"), the Output printed out "All packages are already installed and there is nothing to restore." But when I checked the solution directory, "packages" folder wasn't restored, and thus the solution couldn't be built.

Start by reproducing the restore from the solution using the shown Directory.Build.props, with StyleCop.Analyzers version 1.1.118 and the removed packages folder. Compare the restore output and resulting solution directory, then verify the solution builds successfully after the package is restored.

Written by the indexing model from the issue text.