Introduce modules loaders and reorganize Woob* classes

Before there were two classes:

• WoobBase, with as argument a path to woob modules, adds it in the PYTHONPATH and load them
• Woob, which uses the woob repositories to install remote modules, and manage ~/.config/woob/backends to persist backends

It is reorganized in two new classes:

• Woob4, which takes as a first argument a "modules loader"
• WoobUserBackends, with the only add is to manage ~/.config/woob/backends to persist backends

So, how to install/load modules, and how you use woob as a stateless or not, is now separated.

There are four classes of modules loaders:

• ModulesLocal, more or less the same than how WoobBase worked, but woob_modules must be in the PYTHONPATH (it doesn't add it anymore)
• ModulesPath, the same but with a specific path for modules
• ModulesWoobRepositories, to install and load modules from https://updates.woob.tech
• ModulesPIP, to install and load modules from PyPI