source: target downloads with corresponding version

Prefer downloading the version of the source package that actually
corresponds to the version of the binary. Should apt-update be
run and a package updated, we do not really want to fetch a newer
copy of the source than that of the binary, we want the exact
corresponding version (kinda the whole point of compiling a source
disc that they correspond). If the exact version is no longer
available then it is surely preferable to list it in the missing
list than end up with a newer version.

Gbp-Dch: Short
Closes: #952932

scripts/build/source_debian

@@ -46,7 +46,15 @@ then
 fi
 
 # Download sources
-Chroot chroot "dpkg --get-selections" | awk '{ print $1 }' > source-selection.txt
+
+cat >> chroot/source-selection.sh << EOF
+#!/bin/sh
+dpkg-query -W -f='\${binary:Package}=\${Version}\n' > source-selection.txt
+EOF
+
+Chroot chroot "sh source-selection.sh"
+rm -f chroot/source-selection.sh
+mv chroot/source-selection.txt ./
 
 echo "${LB_BOOTLOADERS}" | \
 while IFS="," read -r BOOTLOADER