You can get that packages folder back, but you might not be happy with the results, since .Net Core projects rely on so many NuGet packages. Mine is hovering around 1 GB.
Per project: References->Nuget dictates what packages are referenced and restored. But, as Eastrall mentioned, the packages folder is now global and located in your user folder: C:\Users\[YourUsername]\.nuget\packages
3.2. Filter by nuget (top left in dialog). Select General
3.3. Ensure Allow NuGet to download missing packages is checked
Close and re-open the solution. NuGet will download the required packages.
Note: the NuGet.Config configuration can also be achieved by executing the following command from the NuGet Package Manager Console (Tools -> NuGet Package Manager -> Package Manager Console):