Fix and improve how missing pkg dependencies are handled from repos.

- Use an array of strings to store the pkgdeps.
- While adding a missing pkgdep, check if it's already in the queue and
  new required version is greater, in that case replace with new one.

--HG--
extra : convert_revision : xtraeme%40gmail.com-20091129021735-5dqfucofny8slks2

lib/plist.c

@@ -265,8 +265,6 @@ xbps_remove_string_from_array(prop_array_t array, const char *str)
 		return errno;
 
 	while ((obj = prop_object_iterator_next(iter)) != NULL) {
-		if (prop_object_type(obj) != PROP_TYPE_STRING)
-			continue;
 		if (prop_string_equals_cstring(obj, str)) {
 			found = true;
 			break;