[schema] load schema from modules names instead of directories
Introspect cubicweb, cubes and apphome using pkgutil to generate the full list
of modules names for loading the schema.
Keep historical behavior and check if source .py file exists if a module is
found using python bytecode file (.pyc and .pyo)
Loading schema from apphome require apphome to be present in sys.path and that
"schema" module resolve to a file located in apphome.
Update migraction tests to explicitely update sys.path when loading schema from
different apps, use a contextmanager for this so it's more readable.
Require updated logilab-common and yams