Python: filter self steps from use-use flow

Factor out use-use flow in order to do this.
Also improve names and comments.

I also wanted to change the types in `difinitionFlowStep`, but
that broke the module instantiation.

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::PhaseDependentFlow<DP::EssaFlow::essaFlowStep/2>::importTimeStep(nodeFrom, nodeTo)
+    DP::PhaseDependentFlow<DP::LocalFlow::localFlowStep/2>::importTimeStep(nodeFrom, nodeTo)
   }
 }