dh_setup_copyright: Don't use the apt cache to find source package names

The OBS builders don't have any source lists set up, so `apt-cache
showsrc` never finds any results. Instead, we can rely on dpkg-query to
extract the info, along with some adjustments to get the correct version
value.

https://phabricator.apertis.org/T9756



Signed-off-by: Ryan Gonzalez <ryan.gonzalez@collabora.com>

debian/changelog

+debhelper (13.11.4+apertis3) apertis; urgency=medium
+
+  * dh_setup_copyright: Don't use the apt cache to find source package names
+
+ -- Ryan Gonzalez <ryan.gonzalez@collabora.com>  Thu, Nov 14 2023 15:12:00 -0600
+
 debhelper (13.11.4+apertis2) apertis; urgency=medium
 
   * dh_setup_copyright: Save source package names for external files

dh_setup_copyright

@@ -126,22 +126,19 @@ sub find_source_package_for_package {
 	my $cached = $packages_to_srcpackages_cache{$package};
 	return @$cached if defined $cached;
 
-	my $stanza = `apt-cache showsrc "$package"`;
-	my ($srcpackage) = $stanza =~ /Package: (.*)/;
-	my ($srcversion) = $stanza =~ /Version: (.*)/;
-	return undef if !defined $srcpackage || !defined $srcversion;
-
-	my ($binpkgs) = $stanza =~ /Binary: (.*)/;
-	for my $binpkg (split /,\s*/, $binpkgs) {
-		my $existing = $packages_to_srcpackages_cache{$binpkg};
-		if (defined $existing) {
-			print STDERR "WARNING: $binpkg is tied to multiple source packages:",
-				" '$existing->[0]' and '$srcpackage'\n";
-		} else {
-			$packages_to_srcpackages_cache{$binpkg} = [$srcpackage, $srcversion];
-		}
-	}
-
+	my ($version, $srcpackage, $srcversion) =
+		`dpkg-query -f '\${Version} \${Source}' -W '$package'`
+		# Note that the source version *might* be in the Source field, so make
+		# sure to extract it if so.
+		=~ /(\S+) (\S+)(?: \((.*)\))?/;
+	$srcpackage ||= $package;
+	$srcversion ||= $version;
+	return undef if !defined $srcversion;
+
+	# Strip off the build suffix.
+	$srcversion =~ s/(apertis(?:\d+))b.*/$1/;
+
+	$packages_to_srcpackages_cache{$package} = [$srcpackage, $srcversion];
 	($srcpackage, $srcversion);
 }