[FEATURE REQUEST] Move platform specific api in gio::unix, gio::win32, etc

[pbor]

https://gitlab.gnome.org/GNOME/glib/-/merge_requests/3892 proposes to split platform specific API in glib in separate gir files.

If that happens, we would probably have to have separate creates like gio-unix.

In preparation for that we should start moving platform specific API to corresponding modules like gio::unix, so that later these can just become re-exports of the platform specific crates.

Even if the plan for glib is dropped, having platform specific api in modules would be nicer than having them in the global namespace and would be consistent with std.