Python: module for import time flow

The logic for separating local flow into _import time_
and _runtime_ was duplicated a few times.
Create a module for it instead, and add a good qldoc.

python/ql/test/experimental/dataflow/module-initialization/localFlow.ql

@@ -12,7 +12,7 @@ module ImportTimeLocalFlowTest implements FlowTestSig {
     // results are displayed next to `nodeTo`, so we need a line to write on
     nodeTo.getLocation().getStartLine() > 0 and
     nodeTo.asVar() instanceof GlobalSsaVariable and
-    DP::importTimeLocalFlowStep(nodeFrom, nodeTo)
+    DP::Separate<DP::EssaFlow::essaFlowStep/2>::importTimeStep(nodeFrom, nodeTo)
   }
 }