Crate reorganization

crates/imap/src/core/mailbox.rs

@@ -1,4 +1,10 @@
-use std::{
+/*
+ * SPDX-FileCopyrightText: 2020 Stalwart Labs Ltd <hello@stalw.art>
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-SEL
+ */
+
+ use std::{
     collections::BTreeMap,
     sync::{Arc, atomic::Ordering},
 };