[pyramid/core] drop fallback import of legacy cubes

[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

[doc] better documentation for "cubicweb-ctl newcube" cubicweb-ctl newcube create cubicweb-<name> in current directory.

[test] make unittest_cwconfig.py does not depend on legacy cube machinery By dropping legacy cube tests in various places.

[devtools/test] drop legacy cube i18n tests test_i18ncube_legacy_layout() is droped and make others tests uses a new style cube "cubicweb_i18ntestcube", so it doesn't depends on legacy cube machinery.

[server/test] make unittest_migractions.py use new style cubes MigrationTC used old cube layout in cubicweb/server/test/data-migractions/cubes, move them to new style cubes as "cubicweb_<cubename>" in "apphome" (cubicweb/server/test/data-migractions/) which is added to PYTHONPATH by CubicWebTC.

[cwconfig] drop _cube_modname Since we drop support for legacy cubes, this is equivalent to _cube_pkgname()

[cwconfig] drop legacy cube importer We don't want to support legacy cubes anymore.

Drop more deprecated code This follows up on changeset a8c1ea390400, in which code deprecated using logilab.common.deprecation got dropped. Now we also drop code deprecated using stdlib's warn(<msg>, DeprecationWarning). Notice that, as a consequence of dropping old/new etypes aliases in cubicweb/schema.py, we drop the import ETYPE_NAME_MAP (no longer needed); but since other modules imported that name from cubicweb.schema, we need to update the import statement to use "cubicweb" directly.

Flake8 cubicweb/cwvreg.py