downloadunpacked: fix setuptools import

setuptools.archive_util is not available unless setuptools.archive_util
have been imported before.

For example:
```python
>>> import setuptools
>>> setuptools.archive_util
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'archive_util'
>>> import setuptools.archive_util
>>> setuptools.archive_util
<module 'setuptools.archive_util' from '/usr/local/lib/python2.7/dist-packages/setuptools/archive_util.pyc'>
```

/reviewed-on nexedi/slapos.recipe.build!3

slapos/recipe/downloadunpacked.py

@@ -31,7 +31,7 @@ import subprocess
 import tarfile
 import zc.buildout
 import tempfile
-import setuptools
+import setuptools.archive_util
 
 is_true = ('false', 'true').index