pytest<4 because of https://github.com/pytest-dev/pytest/issues/4641

when this lister is called for the first time, db_num_entries() may return
a null value, so the min() will also be zero, making the range() call crash.

which it won't be when the lister is called for the first time (since
the run() method will be be called with min_bond=None in this case).

so that we can easily manage its configuration (especially in the docker
environment) by referring to this lister as only 'bitbucket' everywhere
(ie. python package name and config file names).

since it's now the default base class in swh-scheduler (>= 0.0.39)

using the host of the given url.

This allows to create a lister task by simply specify the API base url
and prevent 'inconsistent by default' behavior, eg. with:

  swh-scheduler task add swh-lister-gitlab-full \
      api_baseurl=https://0xacab.org/api/v4

the created task does not use 'gitlab' as instance name (but '0xacab.org'
here).

It's still possible to explicitely specify the instance name if needed.

in order to be able to run unit tests using celery pytest fixtures, we
use a dedicated swh_app fixture that ensure the "main" celery app
is the test app (otherwise subtasks won't work).

- use a generator instead of a while loop
- declare a local logger and use it instead of the root logger.

Get rid of the class based task definition in favor of decorator-based
task declarations.

Doing so, we can get rid of core/tasks.py

Task names are explicitely set to keep compatibility with task
definitions in schedulers' database.

This also add debug statements at the beginning and end of each lister
task.

the default implementation of SQLAlchemy's declarative API should
work just fine.

it looks unused.

This new lister enables to get only new or updated npm packages
since the last listing operation.

Related T1378
Closes T1398

Related T1378
Closes T1380

related to T1330

Part of T1261

Prior to this, when wrong url injection happens, no information is
fetched because we have bad input in the first place (404, 400).  This
makes the debugging session quite hard.

Only the main gitlab.com instance supports a rate limit (thus the
credentials need), others do not (as per summer 2018).

related to T1242