Fix importing of C-extensions when people are running from source with a PYTHONPATH containing conflicting pre-imported modules

src/calibre/constants.py

@@ -160,6 +160,10 @@ class Plugins(collections.Mapping):
         if name in self._plugins:
             return
         sys.path.insert(0, sys.extensions_location)
+        try:
+            del sys.modules[name]
+        except KeyError:
+            pass
         try:
             p, err = importlib.import_module(name), ''
         except Exception as err: