[cwconfig] Look for migration scripts directory relatively to code Thus getting rid of the last piece of data in "shared" directory. As a consequence, we can drop data_files from __pkginfo__.py. Related to #17132990.

etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances