tools: fix usage of aliases.json

Load aliases.json from mbed-os rather than from a temporary
directory.

Author: c1728p9

tools/arm_pack_manager/__init__.py

@@ -357,7 +357,7 @@ def aliases(self) :
 
         """
         if not self._aliases :
-            with open(join(self.data_path, "aliases.json")) as i :
+            with open(LocalPackAliases) as i :
                 self._aliases = load(i)
         return self._aliases