[cwconfig] drop importing legacy cubes
This finally drop support for legacy cubes.
In cwconfig, don't load modules in the "cubes" namespace.
In cube_pkginfo() handle CW_MIGRATION_MAP which wasn't working with new style cube.
Drop all method specific to legacy cubes importer: cubes_search_path(), extrapath(), cls_adjust_sys_path()
Drop environment variables related to legacy cubes: CW_CUBES_PATH and CW_CUBES_DIR
^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
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