Before this change, if you wanted to include physis you had to include
a bunch of dependencies you didn't need. For example, a launcher would
have to pull in texpresso which is completely useless. By default,
the features from before (excluding game_install) are included.
Also this creates a game_install feature for eventual Windows builds,
where unshield is not supported.
Removes another crate dependency, and it's only used in one place in the
code. Will eventually pick up the TODO and cut more bits off when
I get around to it.
