python: make DataFlowCall "publicly usable"

- add `getCallable`, `getArg` and `getNode`
- these are `none` for summary calls
- revert "external" uses (they had been changed to `DataFlowSourceCall`)

python/ql/src/Security/CWE-020-ExternalAPIs/ExternalAPIs.qll

@@ -69,7 +69,7 @@ class ExternalApiDataNode extends DataFlow::Node {
   int i;
 
   ExternalApiDataNode() {
-    exists(DataFlowPrivate::DataFlowSourceCall call |
+    exists(DataFlowPrivate::DataFlowCall call |
       exists(call.getLocation().getFile().getRelativePath())
     |
       callable = call.getCallable() and

python/ql/src/meta/analysis-quality/CallGraph.ql

@@ -11,6 +11,6 @@
 import python
 import semmle.python.dataflow.new.internal.DataFlowPrivate
 
-from DataFlowSourceCall c, DataFlowCallableValue f
+from DataFlowCall c, DataFlowCallableValue f
 where c.getCallable() = f
 select c, "Call to $@", f.getScope(), f.toString()