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/modem.py b/src/osmo_gsm_tester/modem.py
index a1e5e97..6759e70 100644
--- a/src/osmo_gsm_tester/modem.py
+++ b/src/osmo_gsm_tester/modem.py
@@ -17,9 +17,10 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-from . import log, util, sms, process
-from .event_loop import MainLoop
+from .core import log, util, process
+from .core.event_loop import MainLoop
 from .ms import MS
+from . import sms
 
 from pydbus import SystemBus, Variant
 import os