[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.
Document: cubicweb-doc
Title: CubicWeb Documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/python-cubicweb-documentation/html/index.html
Files: /usr/share/doc/python-cubicweb-documentation/html/*