Fix possible double import of cubes modules
When cubes using the new layout are imported with 'cubicweb_<cube>' and with
'cubes.<cube>', the same module is imported twice.
Handle this by adding 'cubes.<cube>' to sys.modules when importing from
'cubicweb_<cube>'.
Move load_module() to a sub class _CubesLoader to share informations computed
in find_modules().
Don't handle subpackages in _CubesImporter and rely on normal import machinery
instead.
Add a test and use unittest from cubicweb.devtools.testlib which resolve to
unittest2 on PY2 with assertLogs() method.
Metadata
--------
.. index::
schema: meta-data;
schema: eid; creation_date; modification_data; cwuri
schema: created_by; owned_by; is; is_instance;
Each entity type in |cubicweb| has at least the following meta-data attributes and relations:
`eid`
entity's identifier which is unique in an instance. We usually call this identifier `eid` for historical reason.
`creation_date`
Date and time of the creation of the entity.
`modification_date`
Date and time of the latest modification of an entity.
`cwuri`
Reference URL of the entity, which is not expected to change.
`created_by`
Relation to the :ref:`users <CWUser>` who has created the entity
`owned_by`
Relation to :ref:`users <CWUser>` whom the entity belongs; usually the creator but not
necessary, and it could have multiple owners notably for permission control
`is`
Relation to the :ref:`entity type <CWEType>` of which type the entity is.
`is_instance`
Relation to the :ref:`entity types <CWEType>` of which type the
entity is an instance of.