Fix backwards iteration in cache hierarchy logic

The recent cache refactoring had caused the hierarchy iteration to
not work correctly. In particular, backwards iteration did not
work over module boundaries like before.

exercise/cache/hierarchy.py

@@ -160,8 +160,17 @@ def _go_to_last(indices: List[int]):
 
     if skip_first:
         # Move the starting index to the previous element
-        if tree_indices[-1] == 0:
-            tree_indices = tree_indices[-1]
+        lastidx = len(tree_indices) - 1
+        while tree_indices[lastidx] == 0:
+            if lastidx < 1:
+                tree_indices = 0
+                break
+            if tree_indices[lastidx - 1] >= 0:
+                # if we are at the first chapter at the current level, move to previous item at
+                # next highest level (or move further up the tree, if not possible)
+                tree_indices.pop()
+                break
+            lastidx -= 1
         else:
             tree_indices[-1] -= 1
             _go_to_last(tree_indices)