[infomanager] container.pluginname returns an empty string (fixes #15566)

This fixes an issue reported @ http://trac.kodi.tv/ticket/15566.

Since we're calling it on a container, the paths is a folder (ends with a slash). In that case the usage of `url.GetFileName()` is wrong and leads to an empty label.

I am not entirely sure if the behavior is like intended. It will now return `plugin.video.youtube` for the container path `plugin://plugin.video.youtube/`

@Montellese, @ronie for review please.