Separate private and public interfaces

[warmwaffles]

This is a common pattern that I've seen that helps convey what is supposed to be public and what is internal and not to be used directly.

I don't know if this is something you are interested in.

With this change it would possible to remove the #define __PHYSICSFS_INTERNAL__ before the internal include since the include directory should be the only directory a user should be including within their project. Of course that would necessitate a change to remove the #error out of the internal include, but that can be another change later.