Lets us keep the generic portions of the compatibility list code
together, and allows us to introduce a type alias that makes it so we
don't need to type out a very long type declaration anymore, making the
immediate readability of some code better.
This has gotten sufficiently large enough to warrant moving it to its
own source files. Especially given it dumps the file_sys headers around
code that doesn't use it for the most part.
This'll also make it easier to introduce a type alias for the
compatibility list, so a large unordered_map type declaration doesn't
need to be specified all the time (we don't want to propagate the
game_list_p.h include via the main game_list.h header).
Eliminates the need to rebuild some source files if the file_util header
ever changes. This also uncovered some indirect inclusions, which have
also been fixed.
The follow-up to e2457418da, which
replaces most of the includes in the core header with forward declarations.
This makes it so that if any of the headers the core header was
previously including change, then no one will need to rebuild the bulk
of the core, due to core.h being quite a prevalent inclusion.
This should make turnaround for changes much faster for developers.
Avoids the need to rebuild multiple source files if the filesystem code
headers change.
This also gets rid of a few instances of indirect inclusions being
relied upon
