metadata: interate over source packages when generating Makefile

All build dependencies are between source packages. Interating over source
rather than binary packages simplifies parts of the code and prepares
further improvement.

As a side effect, this changes the implicit default variant of a few
packages (the first defined is used now instead of the lexicographically
first).

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

scripts/metadata.pm

@@ -213,6 +213,7 @@ sub parse_package_metadata($) {
 			$subdir =~ s/^package\///;
 			$subdir{$src} = $subdir;
 			$srcpackage{$src} = {
+				makefile => $makefile,
 				packages => [],
 				buildtypes => [],
 				builddepends => [],