qpkg: change prerequisite resolution order to be fully depth-first

Until now, when considering adding a package, we processed its
dependencies after completing the current list of dependencies. E.g.,
if we had a package A that depended on B and C, B depended on D, and C
depended on E, then the sequence would have been A, B, C, ...

We now process the dependencies of a package immediately after
considering the package, so the sequence above would become A, B, D,
C, ...

The advantage of the new order is that it becomes easier to follow the
dependency tree, which will be beneficial for loop detection and for
ordering packages by installation order.

- prereq.c (resolve): change prerequisite resolution order such that the
  dependencies of the package being considered are processed immediately,
  instead of deferring them until the end of the current dependency list
- prereq.c (prereq): we can now pass the list of dependencies directly,
  without needing a list of lists element
- test/resorder: test resolution order

qpkg/prereq.c

@@ -210,12 +210,11 @@ static void resolve(struct list *next_dep, const struct ref *dep,
 			continue;
 		level++;
 		append_install(pkg);
-		more_deps.refs = pkg->depends;
+		more_deps.refs = dep->next;
 		more_deps.next = next_dep;
 		more_conf.refs = pkg->conflicts;
 		more_conf.next = conf;
-		resolve(&more_deps, dep->next, &more_conf);
-		next_dep = more_deps.next;
+		resolve(&more_deps, pkg->depends, &more_conf);
 		backtrack();
 		level--;
 	}
@@ -224,13 +223,8 @@ static void resolve(struct list *next_dep, const struct ref *dep,
 
 struct pkg **prereq(struct pkg *pkg)
 {
-	struct list deps = {
-		.refs = pkg->depends,
-		.next = NULL
-	};
-
 	/* @@@ make list of pre-existing conflicts */
-	resolve(&deps, NULL, NULL);
+	resolve(NULL, pkg->depends, NULL);
 	free(installs);
 	return best;
 }

qpkg/test/resorder

@@ -0,0 +1,40 @@
+#!/bin/sh
+. ./Common
+
+###############################################################################
+
+qpkg "resolution order" prereq A <<EOF
+Package: A
+Version: 0
+Architecture: test
+Depends: B, C
+Filename: A
+
+Package: B
+Version: 0
+Architecture: test
+Depends: D
+Filename: B
+
+Package: C
+Version: 0
+Architecture: test
+Depends: E
+Filename: C
+
+Package: D
+Version: 0
+Architecture: test
+Filename: D
+
+Package: E
+Version: 0
+Architecture: test
+Filename: E
+EOF
+expect <<EOF
+B
+D
+C
+E
+EOF