Heavy refactor of the task system

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.

swh/lister/pypi/tasks.py

@@ -2,19 +2,18 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
-from ..core.tasks import ListerTaskBase
+from swh.scheduler.task import SWHTask
+from swh.scheduler.celery_backend.config import app
+
 from .lister import PyPILister
 
 
-class PyPIListerTask(ListerTaskBase):
-    """Full PyPI lister (list all available origins from the api).
+@app.task(name='swh.lister.pypi.tasks.PyPIListerTask',
+          base=SWHTask, bind=True)
+def pypi_lister(self, **lister_args):
+    self.log.debug('%s(), lister_args=%s' % (
+        self.name, lister_args))
+    PyPILister(**lister_args).run()
+    self.log.debug('%s OK' % (self.name))
 
-    """
-    task_queue = 'swh_lister_pypi_refresh'
 
-    def new_lister(self):
-        return PyPILister()
-
-    def run_task(self):
-        lister = self.new_lister()
-        lister.run()