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/powersupply.py b/src/osmo_gsm_tester/powersupply.py
index 1cf7106..4a9205d 100644
--- a/src/osmo_gsm_tester/powersupply.py
+++ b/src/osmo_gsm_tester/powersupply.py
@@ -18,8 +18,8 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 from abc import ABCMeta, abstractmethod
-from . import log
-from .event_loop import MainLoop
+from .core import log
+from .core.event_loop import MainLoop
 
 class PowerSupply(log.Origin, metaclass=ABCMeta):