Try relative imports of tahu files if global import fails

Removes the need to add tahu to the PYTHONPATH

enki.py

@@ -5,7 +5,10 @@
 import argparse
 import signal
 
-from sparkplug_b_pb2 import Payload
+try:
+    from sparkplug_b_pb2 import Payload
+except ImportError:
+    from tahu.python.core.sparkplug_b_pb2 import Payload
 
 from mqtt_if import MQTTInterface
 from sp_topic import SPTopic

shell.py

@@ -5,8 +5,14 @@
 
 import cmd2
 
-import sparkplug_b_pb2
-from sparkplug_b import MetricDataType, addMetric, addNullMetric, initDatasetMetric
+try:
+    import sparkplug_b_pb2
+    from sparkplug_b import MetricDataType, addMetric
+    from sparkplug_b import addNullMetric, initDatasetMetric
+except ImportError:
+    import tahu.python.core.sparkplug_b_pb2
+    from tahu.python.core.sparkplug_b import MetricDataType, addMetric
+    from tahu.python.core.sparkplug_b import addNullMetric, initDatasetMetric
 
 import sp_helpers
 from sp_helpers import MsgType

sp_helpers.py

@@ -4,7 +4,10 @@
 from typing import Optional
 from enum import Enum
 
-from sparkplug_b import MetricDataType
+try:
+    from sparkplug_b import MetricDataType
+except ImportError:
+    from tahu.python.core.sparkplug_b import MetricDataType
 
 
 # Sparkplug namespace used