fix DebugWipeLoader being used for zipimporter

Summary:
X-link: facebookincubator/zstrong#664

Need to insure DebugWipeLoader doesn't get used for zipimporter files.  The original imp.find_module that I replaced did not support zipimporter the new importlib find_spec does find the them.

Reviewed By: itamaro

Differential Revision: D52556833

fbshipit-source-id: bf5892a950a965bb3b17ab951910db2e33b69a34

build/fbcode_builder/CMake/fb_py_test_main.py

@@ -120,7 +120,8 @@ def get_code(self, fullname):
                     code_impl.ob_size = 0
                 return code
 
-        spec.loader = DebugWipeLoader(fullname, spec.origin)
+        if isinstance(spec.loader, SourceFileLoader):
+            spec.loader = DebugWipeLoader(fullname, spec.origin)
         return spec