move exchange module content to exchange package and the interface to a new module

freqtrade/exchange/__init__.py

@@ -4,8 +4,8 @@ from typing import List
 
 import arrow
 
-from freqtrade.exchanges import Exchange
-from freqtrade.exchanges.bittrex import Bittrex
+from freqtrade.exchange.bittrex import Bittrex
+from freqtrade.exchange.interface import Exchange
 
 logger = logging.getLogger(__name__)
 

freqtrade/exchange/bittrex.py

@@ -5,7 +5,7 @@ import arrow
 import requests
 from bittrex.bittrex import Bittrex as _Bittrex
 
-from freqtrade.exchanges import Exchange
+from freqtrade.exchange.interface import Exchange
 
 logger = logging.getLogger(__name__)