Python: Distinguish between Python 2 and 3

Also moves the filtering on `name` to before the big disjunction in
`MkModuleImport`.

python/ql/test/experimental/dataflow/ApiGraphs-py2/options

@@ -0,0 +1 @@
+semmle-extractor-options: --lang=2 --max-import-depth=1

python/ql/test/experimental/dataflow/ApiGraphs-py2/test.py

@@ -0,0 +1,3 @@
+def python2_style():
+    from __builtin__ import open #$ use=moduleImport("builtins").getMember("open")
+    open("hello.txt") #$ use=moduleImport("builtins").getMember("open").getReturn()

python/ql/test/experimental/dataflow/ApiGraphs-py2/use.ql

@@ -0,0 +1,30 @@
+import python
+import semmle.python.dataflow.new.DataFlow
+import TestUtilities.InlineExpectationsTest
+import semmle.python.ApiGraphs
+
+class ApiUseTest extends InlineExpectationsTest {
+  ApiUseTest() { this = "ApiUseTest" }
+
+  override string getARelevantTag() { result = "use" }
+
+  private predicate relevant_node(API::Node a, DataFlow::Node n, Location l) {
+    n = a.getAUse() and l = n.getLocation()
+  }
+
+  override predicate hasActualResult(Location location, string element, string tag, string value) {
+    exists(API::Node a, DataFlow::Node n | relevant_node(a, n, location) |
+      tag = "use" and
+      // Only report the longest path on this line:
+      value =
+        max(API::Node a2, Location l2 |
+          relevant_node(a2, _, l2) and
+          l2.getFile() = location.getFile() and
+          l2.getStartLine() = location.getStartLine()
+        |
+          a2.getPath()
+        ) and
+      element = n.toString()
+    )
+  }
+}

python/ql/test/experimental/dataflow/ApiGraphs/test.py

@@ -136,5 +136,6 @@ def obscured_print():
     p("Can you see me?") #$ use=moduleImport("builtins").getMember("print").getReturn()
 
 def python2_style():
-    from __builtin__ import open #$ use=moduleImport("builtins").getMember("open")
-    open("hello.txt") #$ use=moduleImport("builtins").getMember("open").getReturn()
+    # In Python 3, `__builtin__` has no special meaning.
+    from __builtin__ import open #$ use=moduleImport("__builtin__").getMember("open")
+    open("hello.txt") #$ use=moduleImport("__builtin__").getMember("open").getReturn()