Replace apt-transport-tor with APT 1.3's Tor or SOCKS5h support

apt (1.3~rc1) unstable; urgency=medium

  * Summary: [...] socks5h (and tor) support [...]
[...]
  * http: auto-configure for local Tor proxy if called as 'tor'

It seems that we could simply use tor:// instead of tor+http:// everywhere, and drop apt-transport-tor. Alternatively, and probably nicer: we could also just use http:// and use APT 1.3’s socks5h support.

Related issues

• Related to #8198 (closed)
• Blocked by #10289 (closed)

Original created by @intrigeri on 11636 (Redmine)