There is no way to create "folders" to create multiple namespaces in a schema.
Procedures and functions are organized into packages. Packages exist in a schema. In a real environment, it would be very unusual to want to create a standalone procedure or function. Virtually every procedure belongs in a package with related methods.
It would seem unusual that you would really want more levels of organization. A procedure or function should only be declared in the package specification if it needs to be called from outside the package-- otherwise, it can simply be a private method in the package. If you're factoring out very localized bits of code that should only be called from one method, you can nest PL/SQL blocks and define procedures within procedures. If you're organizing things well, most of your code is probably private methods in a package. If you find yourself drowning in many similarly named packages, I would tend to suspect that you need to reconsider how your code is organized.