[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.

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