Python: Limit self argument for PotentialLibraryCall

Using the object from `MethodCallNode` meant that in the code below,
`lib` from the import expression would be considered a self argument

(this showed up in dataflow-consistency query results, that were not
comitted... sorry)

```
from lib import func
func()
```

python/ql/lib/semmle/python/dataflow/new/internal/DataFlowDispatch.qll

@@ -1311,7 +1311,7 @@ class PotentialLibraryCall extends ExtractedDataFlowCall, TPotentialLibraryCall
     // potential self argument, from `foo.bar()` -- note that this could also just be a
     // module reference, but we really don't have a good way of knowing :|
     apos.isSelf() and
-    result = any(MethodCallNode mc | mc.getFunction().asCfgNode() = call.getFunction()).getObject()
+    result.asCfgNode() = call.getFunction().(AttrNode).getObject()
   }
 
   override ControlFlowNode getNode() { result = call }

python/ql/lib/semmle/python/dataflow/new/internal/DataFlowPublic.qll

@@ -348,7 +348,7 @@ class ExtractedArgumentNode extends ArgumentNode {
     normalCallArg(_, this, _)
     or
     // and self arguments
-    this = any(MethodCallNode mc).getObject()
+    this.asCfgNode() = any(CallNode c).getFunction().(AttrNode).getObject()
   }
 
   final override predicate argumentOf(DataFlowCall call, ArgumentPosition pos) {