[web] Add missing import of _DATA_DIR in webctl Followup on d84bc85f7f70 which introduced usage of this name in _datadirs method of GenStaticDataDirMixIn but resulted into a NameError at runtime and makes migrations crash.

mock
psycopg2
ldap3 < 2
cubicweb-basket
cubicweb-card
cubicweb-comment
cubicweb-file
cubicweb-localperms
cubicweb-tag