pallet-staking: Reorg migration, pallet and pallet impls (#9410)

* Reorg migration and pallet and pallet impls

* Fix imports in untouched modules

* Add file headers

* Add header for migrations

* Improve comment

* Move OnOffenceHandler impl for Pallet to impl.rs

* fmt

substrate/frame/staking/src/mock.rs

@@ -22,7 +22,10 @@ use crate::*;
 use frame_election_provider_support::onchain;
 use frame_support::{
 	assert_ok, parameter_types,
-	traits::{Currency, FindAuthor, Get, OnInitialize, OneSessionHandler},
+	traits::{
+		Currency, FindAuthor, GenesisBuild, Get, Hooks, Imbalance, OnInitialize, OnUnbalanced,
+		OneSessionHandler,
+	},
 	weights::constants::RocksDbWeight,
 };
 use sp_core::H256;