We can simplify some of the repository code since there's now two
separate functions for base/expansion repositories. Added tests to make
sure reading doesn't regress like it did before.
In Hotspot it was revealed that a huge chunk of performance is lost due
to repeated IndexFile::from_existing calls, which may be on the same
index file. Now GameData transparently keeps a cache of these in memory
which speeds up the slowest link here.
Currently, we not handle repairing version files well, so I
sought to seek a solution to the problem. This is one of part of the
puzzle, by implementing a simple "game repair". This is not
IndexedZiPatch support, but simply checking if we can restore version
files if possible, or otherwise wipe the directory.
