Move backend source to server/ subpackage

To clarify the organization.

server/src/infra/mod.rs

@@ -0,0 +1,12 @@
+pub mod auth_service;
+pub mod cli;
+pub mod configuration;
+pub mod db_cleaner;
+pub mod graphql;
+pub mod jwt_sql_tables;
+pub mod ldap_handler;
+pub mod ldap_server;
+pub mod logging;
+pub mod sql_backend_handler;
+pub mod tcp_backend_handler;
+pub mod tcp_server;