Make the support for unencrypted Wii disc images less broken

These disc images are only used on dev units and not retail units.
There are two important differences compared to normal Wii disc images:

- The data starts 0x8000 bytes into each partition instead of 0x20000
- The data of a partition is stored unencrypted and contains no hashes

Our old implementation was just guesswork and doesn't work at all.
According to testing by GerbilSoft, this commit's implementation
is able to read and extract files in the filesystem correctly,
but the tested game still isn't able to boot. (It's thanks to their
info about unencrypted disc images that I was able to make this commit.)

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