Fix duplicate creation of common dependencies

If a task was depended on more than once we previously did not
correctly identify this and instead created it more than once.

Change the `add' function such that we correctly hoist the
collection of so-far created elements and the cycle sentinel
outside of the first recursive call. This ensures they survive
the recursion, rather than being re-created at each level.

This undoes the breakage caused by ada4ff9.

Fixes #10

Author: PeterJCLaw

scripts/import.py

@@ -86,26 +86,30 @@ def add(element: str, backend: 'Backend', year: str) -> int:
     We recurse into all the dependencies of the task we're adding, working to
     add the leaves first so that the higher level tasks can be created with the
     links to their dependencies in place from the start.
+
+    We keep a track of the tasks we've imported so far in order that we can
+    detect (an reject) cycles, as well as to ensure that we cross-link any
+    dependencies which have already been imported at the point they are
+    depended upon by a new parent.
     """
     CYCLE = object()
     elements: Dict[str, Union[int, object]] = {}
 
-    if element in elements:
-        previous = elements[element]
-        if previous is CYCLE:
-            raise RuntimeError(f"cyclic dependency on {element}")
-        assert isinstance(previous, int)
-        return previous
-    else:
-        elements[element] = CYCLE
-        generated = process(
-            element,
-            year=year,
-            handle_dep=lambda x: add(x, backend, year),
-        )
-        ticket_id = backend.submit(generated)
-        elements[element] = ticket_id
-        return ticket_id
+    def _add(element: str) -> int:
+        if element in elements:
+            previous = elements[element]
+            if previous is CYCLE:
+                raise RuntimeError(f"cyclic dependency on {element}")
+            assert isinstance(previous, int)
+            return previous
+        else:
+            elements[element] = CYCLE
+            generated = process(element, year=year, handle_dep=_add)
+            ticket_id = backend.submit(generated)
+            elements[element] = ticket_id
+            return ticket_id
+
+    return _add(element)
 
 
 def parse_args() -> argparse.Namespace: