qpkg: moved comp_versions from prereq.c to id.c

- id.c: added section titles
- prereq.c (comp_versions), id.h, id.c: moved comp_versions from prereq.c
  to id.c, to allow sharing with other code

qpkg/id.h

@@ -68,6 +68,7 @@ struct id {
 
 
 int comp_id(const void *a, const void *b);
+int comp_versions(const struct id *va, const struct id *vb);
 
 struct tree *make_tree(int (*comp)(const void *a, const void *b));
 struct jrb *make_id(struct tree *tree, const char *s, size_t len);