refac(exch:offline) move offline exchange to its own package

6e7a12dc · Brian Tiger Chow · ddf889d8 · 6e7a12dc · 6e7a12dc
--- a/exchange/bitswap/offline.go
+++ b/exchange/bitswap/offline.go
--- a/exchange/bitswap/offline_test.go
+++ b/exchange/bitswap/offline_test.go