prefer-export-from should not report when casting the type

[pcorpet]

When enabling the prefer-export-from I get a warning on a valid case: type casting.

For instance in my app I want modules to use my version of redux' useDispatch as I've enforced a different type (narrower).

import {useDispatch as reduxUseDispatch} from 'react-redux'
type MyDispatchType = Dispatch<MyActions>

export const useDispatch: () => DispatchAllActions = reduxUseDispatch

As of v38.0.0, eslint would complain on the last line although it's not equivalent to an export from statement.

[fisker]

#1580