WIP core: rely on namespace packages for importing modules

This has the advantage that modules can be imported just like any standard Python module. Abstract* will simply become useless

TODO:

• add a dependency system so that installing a module will install its dependencies
• play nice with sources.list
• weboob: remove Abstract*
• modules: import other modules instead of using PARENT etc.
• rename modules/ to weboob_modules/
• check compatibility with current installs