modules management: fix, enh&clean of load/import module functions.

Fix: now possible to import as true packages the shinken modules. 
Even if they do relative import. So we don't over populate sys.path with every module path.

Enh: Daemon: set modulesctx.modules_dir when modules_manager is loaded.

Clean: ModulesContext.get_module() and ModulesManager.load().
Factorization should be easily do-able.

It's not so common at all. Such thing do not follow the python was, with explicit> implicit. This looks like perl for me, so we should avoid it.

If we should drop some python features because they sucks, we will do. And I'll refuse any pr that add complexity without a real user gain. You may be used to this else, but it's very unusual, at least in all the python project I read the code :)