[schema] use explicit imports in schema declaration

schema.py

 # template's specific schema
-Expense = import_erschema('Expense')
-Expense.add_relation(SubjectRelation('Workcase', cardinality='?*'), name='spent_for')
+from yams.buildobjs import SubjectRelation
 
@@ -4,1 +3,5 @@
 
+from cubes.expense.schema import Expense
+
+Expense.add_relation(SubjectRelation('Workcase', cardinality='?*'),
+                     name='spent_for')