Python: Make import private for better auto-complete

With the non-private imports, auto-completing on `API::` gave ALL results
available from `import python`, as well as the ones specified in the `API`
module.

The non-private import in Attributes.qll did the same for `DataFlow::`.

python/ql/src/semmle/python/ApiGraphs.qll

@@ -6,7 +6,7 @@
  * directed and labeled; they specify how the components represented by nodes relate to each other.
  */
 
-import python
+private import python
 import semmle.python.dataflow.new.DataFlow
 
 /**

python/ql/src/semmle/python/dataflow/new/internal/Attributes.qll

@@ -3,6 +3,7 @@
 import DataFlowUtil
 import DataFlowPublic
 private import DataFlowPrivate
+private import semmle.python.types.Builtins
 
 /**
  * A data flow node that reads or writes an attribute of an object.
@@ -84,8 +85,6 @@ private class AttributeAssignmentAsAttrWrite extends AttrWrite, CfgNode {
   override string getAttributeName() { result = node.getName() }
 }
 
-import semmle.python.types.Builtins
-
 /** Represents `CallNode`s that may refer to calls to built-in functions or classes. */
 private class BuiltInCallNode extends CallNode {
   string name;