Move EraIndex to sp-staking (#10671)

* Move `EraIndex` to sp-staking

* Fix imports in mocks

* Run cargo +nightly-2021-10-29 fmt

substrate/frame/staking/src/slashing.rs

@@ -50,8 +50,8 @@
 //! Based on research at <https://research.web3.foundation/en/latest/polkadot/slashing/npos.html>
 
 use crate::{
-	BalanceOf, Config, EraIndex, Error, Exposure, NegativeImbalanceOf, Pallet, Perbill,
-	SessionInterface, Store, UnappliedSlash,
+	BalanceOf, Config, Error, Exposure, NegativeImbalanceOf, Pallet, Perbill, SessionInterface,
+	Store, UnappliedSlash,
 };
 use codec::{Decode, Encode};
 use frame_support::{
@@ -63,7 +63,7 @@ use sp_runtime::{
 	traits::{Saturating, Zero},
 	DispatchResult, RuntimeDebug,
 };
-use sp_staking::offence::DisableStrategy;
+use sp_staking::{offence::DisableStrategy, EraIndex};
 use sp_std::vec::Vec;
 
 /// The proportion of the slashing reward to be paid out on the first slashing detection.