Create core directory to contain most of code not in object classes used by tests

Change-Id: I9aec6c55ccd71894182057d36e0025b69925d314
diff --git a/src/osmo_gsm_tester/bts_nanobts.py b/src/osmo_gsm_tester/bts_nanobts.py
index 4cdb98e..539e5ed 100644
--- a/src/osmo_gsm_tester/bts_nanobts.py
+++ b/src/osmo_gsm_tester/bts_nanobts.py
@@ -20,9 +20,10 @@
 import os
 import re
 import json
-from . import log, config, util, process, pcap_recorder, bts, pcu
+from .core import log, config, util, process
+from .core.event_loop import MainLoop
+from . import pcap_recorder, bts, pcu
 from . import powersupply
-from .event_loop import MainLoop
 
 class NanoBts(bts.Bts):