Bug#775136: sources.list.udeb generator doesn't consider Dir::Etc::SourceList

[Aurelien Jarno <aurel32@debian.org>]

Package: debian-installer
Version: 20150107
Severity: important

build daemons traditionally use the Dir::Etc::SourceList apt 
configuration option in order to specify a different sources.list
file. debian-installer sources.list.udeb generator doesn't consider
this option, which causes a FTBFS on some build daemons without
/etc/apt/sources.list or with an outdated one.

This is reproducible on an installation using only the standard
/etc/apt/sources.list:

# echo 'Dir::Etc::SourceList "sources.list2";' > /etc/apt/apt.conf.d/sources.conf
# mv /etc/apt/sources.list /etc/apt/sources.list2
# apt-get -q -q update ; echo $?
0

So it means apt is able to work correctly in this situation.
Unfortunately the debian-installer sources.list.udeb generator doesn't
consider Dir::Etc::SourceList, leading to the following source file:

| make[2]: Entering directory '/home/aurel32/debian-installer-20150107/build'
| Using generated sources.list.udeb:
|    deb [trusted=yes] copy:/home/aurel32/debian-installer-20150107/build/ localudebs/
| make[9]: 'sources.list.udeb' is up to date.
| Ign copy: localudebs/ InRelease
| Ign copy: localudebs/ Release.gpg
| Ign copy: localudebs/ Release
| Get:1 copy: localudebs/ Packages [20 B]
| Ign copy: localudebs/ Translation-en
| Fetched 20 B in 0s (1052 B/s)

-- System Information:
Debian Release: 8.0
  APT prefers unstable
  APT policy: (500, 'unstable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 3.16.0-4-amd64 (SMP w/8 CPU cores)
Locale: LANG=fr_FR.UTF-8, LC_CTYPE=fr_FR.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)