[IMP] improve overridden sql constraints support

If module A define an sql constraints and module B, which depends on 
module A, override the sql constraint, we must ensure having only the 
last definition in constraints list. Having the two versions in the list
makes the ORM continuously trying to drop the second version by the 
first one, then recreate the second version, when performing operations 
like an upgrade a module B.

An illustration of this is the login_key on res.users, which is unique
per login in base, but unique per (login, website) in website.

closes odoo/odoo#30334

odoo/models.py

@@ -519,7 +519,7 @@ class BaseModel(MetaModel('DummyModel', (object,), {'_register': False})):
         cls._inherits = {}
         cls._depends = {}
         cls._constraints = {}
-        cls._sql_constraints = []
+        cls._sql_constraints = {}
 
         for base in reversed(cls.__bases__):
             if not getattr(base, 'pool', None):
@@ -540,10 +540,12 @@ class BaseModel(MetaModel('DummyModel', (object,), {'_register': False})):
                 # cons may override a constraint with the same function name
                 cls._constraints[getattr(cons[0], '__name__', id(cons[0]))] = cons
 
-            cls._sql_constraints += base._sql_constraints
+            for cons in base._sql_constraints:
+                cls._sql_constraints[cons[0]] = cons
 
         cls._sequence = cls._sequence or (cls._table + '_id_seq')
         cls._constraints = list(cls._constraints.values())
+        cls._sql_constraints = list(cls._sql_constraints.values())
 
         # update _inherits_children of parent models
         for parent_name in cls._inherits: