solver: preference scoring

Should now choose packages better if the best available version
is uninstallable for some reason.

src/test.c

@@ -101,7 +101,7 @@ static void print_dep_errors(char *label, struct apk_dependency_array *deps)
 		struct apk_dependency *dep = &deps->item[i];
 		struct apk_package *pkg = dep->name->state_ptr;
 
-		if (pkg != NULL && apk_dep_is_satisfied(dep, pkg))
+		if (apk_dep_is_satisfied(dep, pkg))
 			continue;
 
 		if (print_label) {

test/error1.expect

-2 unsatisfiable dependencies (solution with 4 names)
-world: d>1.5
-c-1: d>1.0
+1 unsatisfiable dependencies (solution with 4 names)
+b-1: d<2.0

test/error2.expect

 1 unsatisfiable dependencies (solution with 4 names)
-c-1: d>1.0
+world: d<1.5

test/error3.expect

-1 unsatisfiable dependencies (solution with 4 names)
-world: !b
+1 unsatisfiable dependencies (solution with 3 names)
+a-3: b

test/error5.expect

-2 unsatisfiable dependencies (solution with 4 names)
-a-3: d>1.5
-c-1: d>1.0
+1 unsatisfiable dependencies (solution with 4 names)
+b-1: d<2.0