gamepacks: install them in a gamepacks/ subdirectory
This not only unclutters the installation dir, it makes cmake job easier at installing files, not having to use GLOB
that requires to run the cmake configure step again between build and install step¹.
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¹note that this is still required for lib bundling, but not required anymore for gamepacks, so half the job is done to get rid of that double cmake configure step.