session: check for already loaded config in rpc_login_test_login

When the rpc_session_thaw function is restoring saved sessions,
rpc_login_test_login is called multiple times on the same UCI
context. It needs to use uci_lookup_package in case configuration is
already loaded. Otherwise, if there is more than one session to
restore, only the first one will succeed and the remaining ones end up
with the ACLs missing.