method for getting webseed url, bugfix v1 single-file torrent creation

[sneakers-the-rat]

By the time we have piece ranges, we don't know about the torrent info.name field anymore, so we can't construct URLs accurately.

Give that responsibility to the relevant piece range classes, giving them a webseed_url method that can be used to get the full url to request from some url that's used as a webseed.

so e.g. for a multi-file torrent named my_torrent with a file a.exe, a webseed given as https://example.com/data/ should have the file stored at https://example.com/data/my_torrent/a.exe

this also fixes v1-only single file torrents (a rare and discouraged case) which improperly added the metadata to the files list, rather than just having name and length.

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📚 Documentation preview 📚: https://torrent-models--8.org.readthedocs.build/en/8/