package.debian: Re-implement debian loader within the package loader mechanism

This introduces the debian package loader.

Contrary to other package loaders, a package can have multiple artifacts (per package version).

In those terms, we had to slightly adapt the swh.loader.package.loader.PackageLoader to demonstrate this. Thus the impacts on pypi, npm, deposit modules as well.

Related #2025 (closed) Related swh-core!91 (closed) Related swh-lister!114 (closed)

Test Plan

tox

---

Migrated from D2135 (view on Phabricator)

swh/loader/package/tests/conftest.py

@@ -3,12 +3,22 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
+import os
 import pytest
-from os import path
 
 
 @pytest.fixture
 def swh_config(monkeypatch, datadir):
-    conffile = path.join(datadir, 'loader.yml')
+    conffile = os.path.join(datadir, 'loader.yml')
     monkeypatch.setenv('SWH_CONFIG_FILENAME', conffile)
     return conffile
+
+
+@pytest.fixture(autouse=True, scope='session')
+def swh_proxy():
+    """Automatically inject this fixture in all tests to ensure no outside
+       connection takes place.
+
+    """
+    os.environ['http_proxy'] = 'http://localhost:999'
+    os.environ['https_proxy'] = 'http://localhost:999'