[pyramid] No more need for an internal connection when user is cached

[web] req.lang should be None or a non-empty string

[web] Simplify a bit language handling Drop `set_default_language` and `set_user_language` by considering that: * before being bound to the user, request will have the site's default language * cnx has the user's preferred language, hence get it back to the request when it's bound to a connection The first point requires some change to cubicweb's vreg faking so it doesn't break. That should be enough.

Fix AttributeError for "lang" on repo/client connections This is a regression introduced by b48020a80dc3, which removed call to set_language from the `req._set_user` method. This is fine, but we still want a language on connections, so we've to handle it if necesary in Connection.__enter__ (i.e. once connection is properly open). When using pyramid front-end, which has a users cache, we've to cache its language as well because we must not access its preferred_language method since it's not bound to a proper connection.

Alias ugettext to gettext method of cwGNUTranslations in Python 3 The former does not exist in Python 3, but we rely on it. This fixes test_login_bad_password in cubicweb/pyramid/test/test_login.py which has never passed since being introduced in 6392f34fcdad.

[devctl] make i18n custom message extractors work with legacy layouts distname needs to be "cubicweb_<cubename>", even with legacy layouts for pkg.load_entry_point() to work closes #16272177