[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.
^build$
^dist$
\.egg-info$
^.tox$
^.cache$
\.pyc$
\.pyo$
\.bak$
\.old$
\~$
\#.*?\#$
\.swp$
^doc/book/en/apidoc$
\.old$
\.pybuild
cubicweb/server/test/data/slapd.conf
debian/python-cubicweb
debian/*.log
debian/*.substvars
debian/cubicweb-doc
debian/cubicweb
debian/files
syntax: regexp
.*/data.*/database/.*
.*/data/ldapdb/.*
.*/data/uicache/
.*/data/libpython/cubicweb_.*/i18n/.*\.po
^doc/html/
^doc/doctrees/
^doc/book/en/devweb/js_api/
^doc/_build
^doc/js_api/
test-results.xml
ve