Change origin type from 'gnu' to 'tar'

They are useless.

Related T1824
Related P438

As phabricator is an "instance" lister (there exists multiple instances of
phabricator in the wild), we need to reference that information.

In effect, this aligns phabricator lister with for example the gitlab one.

Related T1801
Related P434

Related P434

Prior to this commit, this expected the api.token to be provided at task
initialization. That behavior has been kept for cli purposes. It's no good for
production purposes though (as this leaks the credentials in the scheduler db).

So now, the credentials is fetched from the lister's configuration file as the
other listers do.

Another change is the authentication mechanism which is slighly different. It's
not using a basic `auth` mechanism. It's expecting an `api.token` query
parameter so the `request_params` is overriden to provide that.

Related T1809

This should have been removed along with the code in b8162127.

The request authentication has been reworked so that all listers use the same
credentials dict.

Related b8162127
Related T1772

Prior to this commit, lister did create origins as well in the archive. Now, we
only schedule new origins for ingestion.

Add a lister to list all the CRAN packages .
It uses the build-in API in R language to list the packages
and get their metadata. 

Closes T1709

Added a function which will derive the extension from filename
and check if the fie extension match the type of file that is to be
archived.

As discussed in T1389 to ingest all packages using base loader, it need
a list of all the tarballs for a pakage.
Hence modifified lister to recursively list all the tarballs for a
package with their last updated time.

Previously gnu lister was using same code as that of tarball loader
to download, unzip and read tree.json file.
To make the code consise the downloading method is changed to
requests library.

Implement first pass of gnu lister to list all the
packages present in https://ftp.gnu.org/
Add GNU lister in README and cli.py

Closes T1722

Some of the new listers like GNU and CRAN do not
follow the conventional way of making an HTTP
request, hence they do not need some of the methods
which are usually needed by in conventional HTTP
request.

But those method are marked abstractmethod in the
core making them necessary to be present. So  it is in
best to remove abstractmethod to increase the
readability of those listers.

This becomes a dictionary of key <lister-name>, value a dict of key
<instance-name>, value list of dict username/password.

Related T1772

also add a cli group named 'lister' for the sake of consistency with
other swh packages and rename the command as 'db-init', like:

  swh lister db-init LISTER [...]

Related T1508

This is (hopefully) a temporary fix that can be removed as soon as
  https://github.com/pypa/pip/issues/6239
is fixed, probably thanks to
  https://github.com/pypa/pip/issues/988

Add a lister of all hosted repositories on a Phabricator instance
Closes T808

the exception can also be an AttributeError.

Also do not reraise this exception (in github/tasks.py). This promise
saving feature is used for tests.

Related T1508

Prior to this commit, initializing all listers was failing after the
debian lister processing because of global insert_minimum_data init

Related T1629

Summary:
Using order by and offset makes the partitioning a n^2 operation on the number
of entries in the table, rather than an instant operation when using
min/max.

This assumes the indexable column is more or less uniform, which is not exactly
true but not the worst approximation either.

Test Plan: tox

Reviewers: #reviewers, douardda

Reviewed By: #reviewers, douardda

Subscribers: douardda, swh-public-ci

Differential Revision: https://forge.softwareheritage.org/D1267

Summary:
Using order by and offset makes the partitioning a n^2 operation on the number
of entries in the table, rather than an instant operation when using
min/max.

This assumes the indexable column is more or less uniform, which is not exactly
true but not the worst approximation either.

Test Plan: tox

Reviewers: #reviewers, douardda

Reviewed By: #reviewers, douardda

Subscribers: douardda, swh-public-ci

Differential Revision: https://forge.softwareheritage.org/D1267