Fix logging handler management when reloading

The main module was removing the `_logging` module without invoking
`plugin_unloaded`, which lead to the module being reloaded without the
previous handler being cleaned up and causing duplicate logging events,
especially when reloading the package with the
AutomaticPackageReloader.

main.py

@@ -1,13 +1,13 @@
 import sys
 
 # clear modules cache if package is reloaded (after update?)
-prefix = __package__ + "."  # don't clear the base package
+prefix = __package__ + ".plugins"  # don't clear the base package
 for module_name in [
     module_name
     for module_name in sys.modules
-    if module_name.startswith(prefix) and module_name != __name__
+    if module_name.startswith(prefix)
 ]:
     del sys.modules[module_name]
-prefix = None
+del prefix
 
 from .plugins import *  # noqa