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

The previous url is correct and redirects to this new one.

Related T422

Related T422

The xmlrpc is marked as deprecated [1]. Even if it's not now, the
legacy api is not marked as deprecated. So moving towards this one
sounds more reasonable [2].

[1] https://warehouse.readthedocs.io/api-reference/xml-rpc/#pypi-s-xml-rpc-methods

[2] https://warehouse.readthedocs.io/api-reference/legacy/#simple-project-api

Related T422

Based solely on pypi's deprecated xmlrpc api [1].  No other way of listing
pypi.org is referenced (except for parsing an html page through a
legacy api [2])

[1] https://warehouse.readthedocs.io/api-reference/xml-rpc/#pypi-s-xml-rpc-methods

[2] https://pypi.python.org/simple/

Related T422