parse_pack.py: make directory sort case-insensitive

[tjanas]

parse_pack currently performs a case-insensitive sort on files within a directory. However, it does a case-sensitive sort on directories (in a Linux environment).

For example, given the following file tree:

.
└── MiSTer Add On Packs
    ├── config
    │   └── MSX.CFG
    └── Games
        └── MSX
            ├── boot.vhd
            ├── MSX1 ROM list.txt
            ├── MSX2 ROM list.txt
            └── readme.txt

parse pack will create the entry for config/MSG.CFG after it creates the entries for Games/. Ideally, it would create the entry for MiSTer Add On Packs/config/MSX.CFG before MiSTer Add On Packs/Games/MSX/boot.vhd