[repoapi] move get_repository function into a new repoapi module This new module aims to host the function of the new API replacing the old DBAPI. `get_repository` is still needed hence moved to the new module. Related to #2503918

[testlib] gather all repository access logic in one place Refactoring of the repository access API in test is imminent. We plan to move from the "old" dbapi to the new repoapi. Gathering all impacted method in one place help to understand how all those method interact and help readability for both patch and resulting code. No code change is done at all in this changeset. The refactoring will code later.

[testlib] move repo and related attribute back on Instance instead of Class The repo and cnx was hold by the TestCase class to work as a cache and avoid recreation of the repo from scratch of each test. However since bad26a22fe29 the caching is done by the DatabaseHandler object and it is not necessary have a second layer of cache on the TestCase itself. We move the repo and cnx attribute back on the TestCase instance itself and make several class methods instance methods again. This will helps to change dbapi access in test from dbapi to repoapi. related to #2503918

[dbapi] makes anonymous_connection a computed property The current implementation is a boolean flag set manually by client code after connection creation. This led to different way to decide a anonymous_connection should be True (eg. different in the test than in the actual application code). It should not be client responsibility to set this flag. ``cnx.anonymous_connection`` is now a purely computed property. Connection with user in the "guests" group are anonymous, the other ain't. ``Session.anonymous_session`` is computed from ``cnx.anonymous_connection`` and get the updated behavior transparently. Closes #2953943

[webrequest] simplify set_session code Thanks to the previous changeset we are assured that session handed to set_session is full featured one. This allows a simpler code for this method. related to #2503918

[webrequest] set DBAPISession without cnx at initialisation time Such session are necessary for minimal use of a Request. Setting on by default allow simplification later linking with a full featured DBAPISession or equivalent. This was not possible before as all session was tracked by session manager. They are not tracked anymore since aa709bc6b6c1 and we can safely create them by default. related to #2503918

[testlib] rework request building in init_authentication The previous code was building a full authenticated request and tried to undo the authentication afterward. The new code just create a bare Request with no authenticated session linked yet. This is much closer of what the actual authentication process does.

[request] drop the user argument for set_session I see no code nor test that use this optional argument. removing it help to clean the session code. The set_session function will soon be deprecated anyway (at the same time than the dbapi) related to #2503918

[web-request] handle default language earlier We now read language negotiation header at initialisation time (previously done during ``set_session``). This simplify the set_session code for later reworking. There is no change in behavior, the user selected language continue to overwrite the one picked from the header/default when applicable. related to #2503918

[etwist] pass the repository to the root resource This is another step toward a cleaner instantiation scheme for the repo. The http test now pass an already created object to the etwist server instead of relying on the config cache. Related to #2249513