[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.
[MASTER]
load-plugins=cubicweb.pylintext
ignore=__pkginfo__
[MESSAGES CONTROL]
disable = too-many-ancestors,too-many-instance-attributes,too-many-public-methods,
too-few-public-methods,too-many-arguments,import-error
[BASIC]
function-rgx = [a-z_][a-z0-9_]{2,35}$
good-names=w,_
[TYPECHECK]
generated-members=debug,info,notice,warning,error,critical,exception
[CLASSES]
exclude-protected=_cw,_cnx,
# namedtuple public API.
_asdict,_fields,_replace,_source,_make
[FORMAT]
max-line-length=100
max-module-lines=2000