ResolveNuGetPackageAssets.ReferencedPackages returns the wrong result when dependencies are listed under framework

[nkolev92]

Sample project:

{
  "dependencies": {
    "System.Runtime": "4.0.0",
    "System.Dynamic.Runtime": "4.0.0",
    "System.Linq.Expressions": "4.0.0"
  },
  "frameworks": {
    "dotnet5.1": {
      "dependencies": {
        "System.IO": "4.0.0"
      }
    }
  }
}

When building this I would expect the build task to return System.IO in the list of ReferencedPackages. It does not.

Potentially hacky fix: dotnet/buildtools#368

Moved from: NuGet/Home#1843

cc @ericstj @jasonmalinowski