let data, maps and mp3 directory be truly case insensitive

A user of the Debian vcmi package [reported](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=771300) that the french version of the GOG installer contains the data, maps and mp3 directories in lower case. `vcmibuilder` was not able to handle this, so this patch makes the matching truly case insensitive to prevent any further issue like this.