De-dupe SegaCD and MegaSD packs.

[hotelvictorcharlie]

I don't really understand the MegaSD pack in its current state. It lives inside the SegaCD pack directory (/Sega CD/MegaSD/) but almost all of its contents are duplicates of discs that also live elsewhere in /Sega CD/ (with the same checksums). So this wastes a ton of storage and none of the other packs work like that (except for the MiSTer add-ons, but the MegaSD isn't a SegaCD add-on).

So, in this PR I have:

• De-duped all of the discs that lived in both directories, and moved the combined results into the SegaCD pack. (The MegaSD versions had better naming and directory structure so I favored them.)
• Left the MD+ music stuff in the MegaSD pack (AFAIK the MegaSD is the only device that supports these files).
• Put placeholders in the MegaSD pack for everything else that the MegaSD supports (Genesis, 32X, Sega CD, and Master System). This kind of depends on the existence of standalone packs for Genesis and Master System, which won't exist unless/until #854 gets merged, but I guess the Mega EverDrive and Master EverDrive packs can be used instead of these for now if this PR gets merged first.
• Renamed a few directories in the Sega CD pack for consistency/correctness: 2 Other Languages => 2 Other Regions 4 Unreleased => 2 Unlicensed - A-Z 5 Sega CD Bios => 5 Sega CD BIOS