fix: Priority.DEFAULT -> Priority.PRIMARY

* it appears that the default priority has been removed
* since this is supposed to be a proxy for what pypi uses, change
  this to primary
* TODO: maybe make this a flag

src/poetry_plugin_pypi_proxy/plugin.py

@@ -109,7 +109,7 @@ def activate(self, poetry: Poetry, io: IO) -> None:
             LegacyProxyRepository(
                 name=proxy_id, url=f"{proxy_url}/simple/", config=poetry.config
             ),
-            priority=Priority.DEFAULT,
+            priority=Priority.PRIMARY,
         )
 
         # If this is a publish command to Pypi, we'll silenly redirect to the proxy