Add XDG compatible location for osc plugins.

I believe it is generally a good idea to move stuff from $HOME to
XDG-compatible directories.

osc/commandline.py

@@ -9570,6 +9570,7 @@ Please submit there instead, or use --nodevelproject to force direct submission.
             '/usr/lib/osc-plugins',
             '/usr/local/lib/osc-plugins',
             '/var/lib/osc-plugins',  # Kept for backward compatibility
+            os.path.expanduser('~/.local/lib/osc-plugins'),
             os.path.expanduser('~/.osc-plugins')]
         for plugin_dir in plugin_dirs:
             if not os.path.isdir(plugin_dir):