basic/hashmap,set: change "internal_" to "_" as the prefix

"internal" is a lot of characters. Let's take a leaf out of the Python's book
and simply use _ to mean private. Much less verbose, but the meaning is just as
clear, or even more.

src/basic/ordered-set.h

@@ -59,7 +59,7 @@ static inline void* ordered_set_steal_first(OrderedSet *s) {
 }
 
 static inline char **ordered_set_get_strv(OrderedSet *s) {
-        return internal_hashmap_get_strv(HASHMAP_BASE((OrderedHashmap*) s));
+        return _hashmap_get_strv(HASHMAP_BASE((OrderedHashmap*) s));
 }
 
 int ordered_set_consume(OrderedSet *s, void *p);