[NuGet] Support custom package directory.

NuGet allows a solution to override the standard packages directory
with a repositoryPath setting in the NuGet.config file at the solution
level in the .nuget directory.

  <configuration>
    <config>
      <add key="repositoryPath"
        value="../../MyPackages" />
    </config>
  </configuration>

This is now supported by the NuGet addin to make it compatible with
Visual Studio.