download: only take basename of 'filename' when the latter is used to calculate destination path

Currently only useful for buildout.hash.cfg files in
https://lab.nexedi.com/nexedi/slapos (e.g. software/slaprunner)

slapos/recipe/download.py

@@ -52,8 +52,11 @@ class Recipe(object):
       shared.assertNotShared("option 'destination' can't be set")
     else:
       self._shared = shared
+      filename = options.get('filename')
       destination = os.path.join(shared.location,
-                                 options.get('filename') or name)
+        # filename may be used to also construct source-related options
+        # and we have values that start with a subfolder
+        os.path.basename(filename) if filename else name)
       # Compatibility with other recipes: expose location
       options['location'] = shared.location
     options['target'] = self._destination = destination