Fix error when installing over existing plugins

As reported in PICARD-444, the following error is received when
installing over an existing plugin:

Traceback (most recent call last):
File ".\picard\ui\options\plugins.py", line 134, in open_plugins
self.install_plugin(path)
File ".\picard\ui\options\plugins.py", line 148, in install_plugin
self.tagger.pluginmanager.install_plugin(path, dest)
File ".\picard\plugin.py", line 198, in install_plugin
same_file = os.path.samefile(path, dest) if dest_exists else False
AttributeError: 'module' object has no attribute 'samefile'