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/lib.rs

@@ -312,7 +312,7 @@ use sp_runtime::{
 };
 use sp_staking::{
 	offence::{Offence, OffenceError, ReportOffence},
-	SessionIndex,
+	EraIndex, SessionIndex,
 };
 use sp_std::{collections::btree_map::BTreeMap, convert::From, prelude::*};
 pub use weights::WeightInfo;
@@ -332,9 +332,6 @@ macro_rules! log {
 	};
 }
 
-/// Counter for the number of eras that have passed.
-pub type EraIndex = u32;
-
 /// Counter for the number of "reward" points earned by a given validator.
 pub type RewardPoint = u32;