Default to setuptools' wheel support instead of failing

Setuptools 38.2.0 suddenly started supporting and downloading wheels.
.whl files would end up in the non-functioning and deprecated unpack_wheel().
The new default ('just use setuptools') works fine.

- If you use an older setuptools, setuptools won't download wheels and the new
  code won't be used.

- If you use buildout.wheel, you'll overwrite the unpacker anyway.

src/zc/buildout/easy_install.py

@@ -1641,7 +1641,7 @@ def unpack_wheel(location, dest):
 
 UNPACKERS = {
     '.egg': unpack_egg,
-    '.whl': unpack_wheel,
+    '.whl': unpack_egg,  # Since 38.2, setuptools handles wheels, too.
 }