fix imports from the wrong module

when module_a.submodule_a imports module_b, then module_c should not use
"from module_a.submodule_a import module_b"

src/calibre/gui2/store/__init__.py

@@ -58,7 +58,7 @@ class StorePlugin(object):  # {{{
         self.name = name
         self.base_plugin = base_plugin
         if config is None:
-            from calibre.gui2 import JSONConfig
+            from calibre.utils.config import JSONConfig
             config = JSONConfig('store/stores/' + ascii_filename(self.name))
         self.config = config