PolicyPackageNotFound: Module rucio.common.permission.generic_multi_vo not found
Hi,

While setting up an instance with multi_vo mode enabled using the default permissions package, I got the following error on the client (just from running rucio whoami):
```
2021-04-26 10:40:51,035 ERROR   Cannot authenticate.
Details: PolicyPackageNotFound: Module rucio.common.permission.generic_multi_vo not found 
```

I think this may just be a simple typo in the fallback package name here:
https://github.com/rucio/rucio/blob/b7102af49b39ed79e3fafa43d82e91620d721be6/lib/rucio/core/permission/__init__.py#L88

Is this meant to be "rucio.core.permission." instead and could it please be fixed?

Regards,
Simon

diff --git a/lib/rucio/core/permission/__init__.py b/lib/rucio/core/permission/__init__.py
index 73d81d70d..02ca47550 100644
--- a/lib/rucio/core/permission/__init__.py
+++ b/lib/rucio/core/permission/__init__.py
@@ -85,7 +85,7 @@ def load_permission_for_vo(vo):
                 POLICY = GENERIC_FALLBACK
             POLICY = 'rucio.core.permission.' + POLICY.lower()
     else:
-        POLICY = 'rucio.common.permission.' + GENERIC_FALLBACK.lower()
+        POLICY = 'rucio.core.permission.' + GENERIC_FALLBACK.lower()
 
     try:
         module = importlib.import_module(POLICY)
['lib/rucio/core/permission/__init__.py']