Eventually, we want to move to using `GType` directly for everything,
since `GIBaseInfo` and its subclasses are all using `GTypeInstance`.

However, that requires quite a lot of changes and we’re about to hit the
API freeze.

So do the smallest set of changes possible to remove `GIInfoType` and
related functions from the public API, which gives us freedom to make
more changes later without breaking API.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3155