plugins: on some systems plugins with relative imports failed to load

this caused electrum to fail to start
potentially only older python 3.6.x are affected

fixes #5421

electrum/plugin.py

@@ -27,6 +27,7 @@ import pkgutil
 import importlib.util
 import time
 import threading
+import sys
 from typing import NamedTuple, Any, Union, TYPE_CHECKING, Optional
 
 from .i18n import _
@@ -73,6 +74,9 @@ class Plugins(DaemonThread):
                 raise Exception(f"Error pre-loading {full_name}: no spec")
             try:
                 module = importlib.util.module_from_spec(spec)
+                # sys.modules needs to be modified for relative imports to work
+                # see https://stackoverflow.com/a/50395128
+                sys.modules[spec.name] = module
                 spec.loader.exec_module(module)
             except Exception as e:
                 raise Exception(f"Error pre-loading {full_name}: {repr(e)}") from e