Turns out all newly listed repositories were filtered out because of that.
Consequently, no entries in the listers database and no scheduler loading
tasks were created when listing a Phabricator instance.

Closes T1999

Related T1984

it is not used anymore.

Since all the listing tasks accepts an url as first argument (whatever the
argument name is), it makes sense to use a simple common argument name for
this. I've chosen 'url' instead of api_baseurl/forge_url/url.

Also kill now useless `new_lister()` functions.

This is needed to fix the db-init implementation so the debian loader (which
does use the SQLBase from swh.storage) have its models declared in the
MetaData used by the initialize() function.

Since the CGit lister now perform an HTTP query for each git repos listed in
the main index, it is significantly slower, so reducing the time between
database commits make sense, and won't overload the database.

With a bit of logging, it makes it easier to follow/debug the progress of
a listing.

Add a new register-task-types cli that will create missing task-type entries in the
scheduler according to:

- only create missing task-types (do not update them), but check that the
  backend_name field is consistent,
- each SWHTask-based task declared in a module listed in the 'task_modules'
  plugin registry field will be checked and added if needed; tasks which name
  start wit an underscore will not be added,
- added task-type will have:
  - the 'type' field is derived from the task's function name (with underscores
    replaced with dashes),
  - the description field is the first line of that function's docstring,
  - default values as provided by the swh.lister.cli.DEFAULT_TASK_TYPE (with
    a simple pattern matching to have decent default values for full/incremental
    tasks),
  - these default values can be overloaded via the 'task_type' plugin registry
    entry.

For this, we had to rename all tasks names (eg. `cran_lister` -> `list_cran`).

Comes with some tests.

Listers are declared as plugins via the `swh.workers` entry_point.

As such, the registry function is expected to return a dict with the
`task_modules` field (as for generic worker plugins), plus:

- `lister`: the lister class,
- `models`: list of SQLAlchemy models used by this lister,
- `init` (optionnal): hook (callable) used to initialize the lister's state
  (typically, create/initialize the database for this lister).
  If not set, the default implementation creates database tables (after
  optionally having deleted exisintg ones) according to models declared in
  the `models` register field.

There is no need for explicitely add lister task modules in the main
`conftest` module, but any new/extra lister to be tested must be registered
(the tested lister module must be properly installed in the test environment).

Also refactor a bit the cli tools:
- add support for the standard --config-file option at the 'lister' group
  level,
- move the --db-url to the 'lister' group,
- drop the --lister option for the `swh lister db-init` cli tool:
  initializing (especially with --drop-tables) the database for a single
  lister is unreliable, since all tables are created using a sibgle MetaData
  (in the same namespace).

This is needed by the (refactored) db init mechanism, since this later uses
the main declarative base class (thus the main MetaData instance) to gather
tables to be created/dropped.

forgot the forge_url -> api_baseurl renaming in there.

forgot some `url_prefix` there.

- use the 'standard' api_baseurl as init argument,
- make it optional, with default to forge.softwareheritage.org,
- use origin_url as id.

and simplify a bit the code of the constructor.

This is required to be able to make lister classes instanciation easier and more
reliable, especially in the context of cli tools like 'swh lister run', for which
we want to be able to specify any lister init argument as extra parameter of the
command.

Simplify the code:
- do only inherit from ListerBase
- implement HTTP queries directly using requests
- get rid of convoluted code

Make the origin_url gathered from the git repo's "project" page instead of
building it from the 'url_prefix' hack. Now, the lister WILL make substancially
more requests, since it will make one request per listed git repo, but
the provided origin_url should be pretty reliable now.

When several url are provided as clonable URLs, choose the http/https one first,
otherwise, choose the first one of the list.

Add proper tests for the cgit lister.

Also, get rid of the 'time_updated' column in the model.

get rid of the "smart" flush_packet_db computation.

instead of picking the first one, so this behavior is consistent with
ListerHttpTransport's one.

and get rid of the unneeded _build_query_params method.

stick to the existing credentials mechanism provided by ListerHttpTransport.

and fix empty values returned by this later (empty list instead of ampty dict).

Example use case:

swh lister run --lister gitlab \
               --priority high \
               --policy oneshot \
               --db-url postgresql://postgres@localhost:5432/swh-listers \
               api_baseurl=https://gitlab.ow2.org/api/v4/

Related T1919

Prior to this commit, the policy and priority were hard-coded.
The default values are now the old hard-coded values.

This will allow to develop a cli to trigger forges listing with oneshot policy
and some priority tasks. Thus ingesting those faster and without manual
interventation as we currently do.

Implement a packagist lister to list the
names and metadata url of all the
packages.

Closes 1776

Add tests for pypi lister
Closes T1890

There were previously no tests for the listers
which are using the class SimpleLister(like pypi)
Refractored test_lister.py of lister core to
accomodate tests for SimpleLister keeping the tests
undisturbed for other lister.