make is_standard_module() properly classify extensions (#659)

Extensions can be installed in either a platform independent or specific
location[1]. The search is updated to include both paths. Previously, an
extension in the platform specific location would be misclassified because
only the independent location was considered and the platform specific location
shared a prefix with the standard lib.

[1] http://docs.python.org/3/distutils/apiref.html#distutils.sysconfig.get_python_lib

Fixes Issue: #658
Co-authored-by: Matt Story <s.matt.story@gmail.com>

astroid/modutils.py

@@ -116,7 +116,7 @@ def _posix_path(path):
         # https://github.com/PyCQA/pylint/issues/712#issuecomment-163178753
         STD_LIB_DIRS.add(_posix_path("lib64"))
 
-EXT_LIB_DIR = get_python_lib()
+EXT_LIB_DIRS = {get_python_lib(), get_python_lib(True)}
 IS_JYTHON = platform.python_implementation() == "Jython"
 BUILTIN_MODULES = dict.fromkeys(sys.builtin_module_names, True)
 
@@ -596,8 +596,9 @@ def is_standard_module(modname, std_path=None):
         # we assume there are no namespaces in stdlib
         return not util.is_namespace(modname)
     filename = _normalize_path(filename)
-    if filename.startswith(_cache_normalize_path(EXT_LIB_DIR)):
-        return False
+    for path in EXT_LIB_DIRS:
+        if filename.startswith(_cache_normalize_path(path)):
+            return False
     if std_path is None:
         std_path = STD_LIB_DIRS
     for path in std_path:

astroid/tests/unittest_modutils.py

@@ -278,7 +278,7 @@ def test_4(self):
 
     def test_custom_path(self):
         datadir = resources.find("")
-        if datadir.startswith(modutils.EXT_LIB_DIR):
+        if any(datadir.startswith(p) for p in modutils.EXT_LIB_DIRS):
             self.skipTest("known breakage of is_standard_module on installed package")
 
         self.assertTrue(modutils.is_standard_module("data.module", (datadir,)))