fix: use default server list for providers that don't have one

There are providers in the provider database
that do not have servers specified.
For such providers default list should be tried
just like when configuring unknown providers.

src/provider.rs

@@ -1,6 +1,6 @@
 //! [Provider database](https://providers.delta.chat/) module.
 
-mod data;
+pub(crate) mod data;
 
 use anyhow::Result;
 use deltachat_contact_tools::EmailAddress;