map file check uses foldernames for files instead of scenario.lua

https://github.com/FAForever/client/blob/ecf3715c9d3851401d82d095dc46fca3edf72f44/src/fa/maps.py#L101-L112

The map files can be named differently than the foldername of the maps, but the check for valid map files only tries the foldername instead of the paths that are stored inside of the scenario.lua file. Maps that differ from that naming convention wont be loaded when selecting them in the hostgame-popup and the default map (Four Corners) will be opened instead. Switching to the map inside of the game lobby (and launching it) works fine.