[cwconfig] Always use "sys.prefix" to determine installation prefix
We drop all legacy logic implemented in _find_prefix() which now happens
to be buggy since we cleaned up setup.py in 3.26 (noticeably because
virtualenv is no longer properly detected).
Closes #17132990.
[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.