extend build_dir test in a way that I think should work but doesn't

tests/build/build_dir.srctree

 PYTHON setup.py build_ext --inplace
 PYTHON -c "import a"
+PYTHON -c "import pkg.b"
 PYTHON check_paths.py
 
 ######## setup.py ########
@@ -11,23 +12,65 @@ from Cython.Build.Dependencies import cythonize
 from distutils.core import setup
 
 setup(
-  ext_modules = cythonize("*.pyx", build_dir="scratch"),
+  ext_modules = (cythonize("*.pyx", build_dir="scratchA") +
+                 cythonize("pkg/*.pyx", build_dir="scratchB")),
 )
 
 ######## a.pyx ########
 
-cdef extern from "a_helper.h":
-    int value
+cdef extern from "helper.h":
+    int value1
     
-assert value == 100
+cdef extern from "subdir/helper.h":
+    int value2
 
-######## a_helper.h ########
+cdef extern from "pkg/pkg_helper.h":
+    int value3
 
-int value = 100;
+assert value1 == 100
+assert value2 == 200
+assert value3 == 300
+
+######## helper.h ########
+
+int value1 = 100;
+
+######## subdir/helper.h ########
+
+int value2 = 200;
+
+######## pkg/__init__.py ########
+
+######## pkg/b.pyx ########
+
+cdef extern from "pkg_helper.h":
+    int value3
+
+cdef extern from "subdir/pkg_helper.h":
+    int value4
+
+assert value3 == 300
+assert value4 == 400
+
+######## pkg/pkg_helper.h ########
+
+int value3 = 300;
+
+######## pkg/subdir/pkg_helper.h ########
+
+int value4 = 400;
 
 ######## check_paths.py ########
 
 import os
-assert os.path.exists("scratch/a.c")
-assert os.path.exists("scratch/a_helper.h")
+assert os.path.exists("scratchA/a.c")
+assert os.path.exists("scratchA/helper.h")
+assert os.path.exists("scratchA/subdir/helper.h")
+assert os.path.exists("scratchA/pkg/pkg_helper.h")
 assert not os.path.exists("a.c")
+
+assert os.path.exists("scratchB/pkg/b.c")
+assert os.path.exists("scratchB/pkg/pkg_helper.h")
+assert os.path.exists("scratchB/pkg/subdir/pkg_helper.h")
+assert not os.path.exists("b.c")
+assert not os.path.exists("pkg/b.c")