For project.json the nuget directory is in the user profile folder (%UserProfile%\.nuget\packages)
UPDATE
From msdn
The global-packages folder is where NuGet installs any downloaded package. Each package is fully expanded into a subfolder that matches the package identifier and version number. Projects using the PackageReference format always use packages directly from this folder. When using the packages.config, packages are installed to the global-packages folder, then copied into the project’s packages folder.
When using PackageReference, the globalPackagesFolder configuration value from nuget.config is used. The default value is:
Windows: %userprofile%\.nuget\packages
Mac/Linux: ~/.nuget/packages
When using packages.config, the repositoryPath configuration value from nuget.config is used. The default value is:
$(Solutiondir)/packages
Both locations (globalPackagesFolder and repositoryPath) can be overridden
using the NUGET_PACKAGES environment variable. The environment variable takes precedence over the configuration setting.