fix: handling of DIRECT returned by Acquire::http:Proxy-Auto-Detect

APT option Acquire::http:Proxy-Auto-Detect allow the use of the DIRECT
keyword to not go through a proxy.
Without this commit, debootstrap try to resolve the host DIRECT. With
it, it bypass http_proxy setup when DIRECT is returned by the script
pointed by Acquire::http:Proxy-Auto-Detect.