split stage / stage2 extraction folders, adds missing zlib dependency as an external repo

[ShizCalev]

STAGE2.dat has some GCX files which have different crc32 hashes from their STAGE.dat versions, and this different data was being lost due to both files being extracted to the same folder.

=== CRC32 DIFFERENCES ===
[DIFF]  init/180720.gcx
        stage: 5FEDCFB1
        stage2: EBCA9E63
[DIFF]  n_title/180720.gcx
        stage: 8329D7A9
        stage2: 0988705C
[DIFF]  select/180720.gcx
        stage: 1053AC60
        stage2: 613AC65B