[schema] drop monkeypatch "name_for" on BaseConstraint
BaseConstraint doesn't have "name_for" method, this is just an addition for
cubicweb needs.
Implement this as a function instead of a monkeypatch to avoid having
"name_for" not defined because cubicweb.schema hasn't been imported.
In particular this fix running cubicweb/server/test/unittest_schema2sql.py::SQLSchemaTC::test_known_values alone.