models: use the same declarative base class for all models

This is needed to fix the db-init implementation so the debian loader (which
does use the SQLBase from swh.storage) have its models declared in the
MetaData used by the initialize() function.

swh/lister/core/models.py

@@ -7,13 +7,14 @@ from datetime import datetime
 import logging
 
 from sqlalchemy import Column, DateTime, Integer, String
-from sqlalchemy.ext.declarative import declarative_base, DeclarativeMeta
+from sqlalchemy.ext.declarative import DeclarativeMeta
 
 from .abstractattribute import AbstractAttribute
 
-logger = logging.getLogger(__name__)
+from swh.storage.schemata.distribution import SQLBase
+
 
-SQLBase = declarative_base()
+logger = logging.getLogger(__name__)
 
 
 class ABCSQLMeta(abc.ABCMeta, DeclarativeMeta):