Add a non-incremental sourceforge lister

Following @zack's work on T735, this change introduces an actual SWH lister for SourceForge.

SourceForge provides a main sitemap that lists sharded sitemaps, which themselves list pages. Each page belongs to a project (or sub-project, though those are rare), information about which can be found by querying a REST API, which gives us the list of any and all VCS used for said project. Both sitemaps and pages have a "last modified" timestamp that will be used in a future patch to implement incremental listing.

More precise information can be found as inline comments or docstrings.

---

Migrated from D5293 (view on Phabricator)

swh/lister/sourceforge/tests/data/adobexmp.json

+{"shortname": "adobexmp", "name": "Extensible Metadata Platform (XMP)", "_id": "4bfa89ecb9363c60a200004f", "url": "https://sourceforge.net/adobe/adobexmp/", "private": false, "short_description": "Adobe's Extensible Metadata Platform (XMP) is a labeling technology that allows you to embed data about a file, known as metadata, into the file itself. ", "creation_date": "2010-05-24", "summary": "", "external_homepage": "", "video_url": "", "socialnetworks": [], "status": "active", "moved_to_url": "", "preferred_support_tool": "", "preferred_support_url": "", "developers": [{"username": "stefanmakswit", "name": "Stefan Makswit", "url": "https://sourceforge.net/u/stefanmakswit/"}, {"username": "samymakki", "name": "Samy Makki", "url": "https://sourceforge.net/u/samymakki/"}, {"username": "frankbiederich", "name": "Frank Biederich", "url": "https://sourceforge.net/u/frankbiederich/"}, {"username": "adobeadmin", "name": "Adobe Admin", "url": "https://sourceforge.net/u/adobeadmin/"}, {"username": "n_oostendorp", "name": "Nathan Oostendorp", "url": "https://sourceforge.net/u/n_oostendorp/"}, {"username": "joergehrlich", "name": "J\u00f6rg Ehrlich", "url": "https://sourceforge.net/u/joergehrlich/"}], "tools": [{"name": "discussion", "mount_point": "discussion", "url": "/adobe/adobexmp/discussion/", "icons": {"24": "images/forums_24.png", "32": "images/forums_32.png", "48": "images/forums_48.png"}, "installable": true, "tool_label": "Discussion", "mount_label": "Discussion"}, {"name": "wiki", "mount_point": "wiki", "url": "/adobe/adobexmp/wiki/", "icons": {"24": "images/wiki_24.png", "32": "images/wiki_32.png", "48": "images/wiki_48.png"}, "installable": true, "tool_label": "Wiki", "mount_label": "Wiki"}, {"name": "wiki", "mount_point": "home", "url": "/adobe/adobexmp/home/", "icons": {"24": "images/wiki_24.png", "32": "images/wiki_32.png", "48": "images/wiki_48.png"}, "installable": true, "tool_label": "Wiki", "mount_label": "Home"}], "labels": ["xmp", "metadata", "adobe"], "categories": {"audience": [], "developmentstatus": [], "environment": [], "language": [], "license": [], "translation": [], "os": [], "database": [], "topic": []}, "icon_url": "https://sourceforge.net/adobe/adobexmp/icon", "screenshots": []}