Well I'm completely perplexed at what this is. I've seen the workitem http://nuget.codeplex.com/workitem/595 that seems to be related to it, but it doesn't seem to be entirely accurate as I'm not sure why it would occur to me.
The Client.MVC.nupkg contains web.config.transform and nothing else, the Client.MVC.nuspec contains a dependency to Client.nupkg.
The Client.nupkg contains 1 dll and 1 pdb file in the lib\net40 folder, the Client.nuspec contains a dependency to Core.nupkg.
The Core.nupkg contains 1 dll and 1 pdb file in the lib\net40 folder, and nothing else, and no dependencies.
I created a new MVC3 Internet application as an entirely new solution, pulled up the package manager console. Set Package Source to my network location source (all Client.nupkg, Core.nupkg and Client.MVC.nupkg are in there) then I run
'Core (≥ 1.0 && < 2.0)' not installed. Attempting to retrieve dependency from source...
Install-Package : External packages cannot depend on packages that target projects.
At line:1 char:16
+ Install-Package <<<< Client.MVC3
+ CategoryInfo : NotSpecified: (:) [Install-Package], InvalidOperationException
+ FullyQualifiedErrorId : NuGetCmdletUnhandledException,NuGet.PowerShell.Commands.InstallPackageCommand