Generating gir and typelib files has inter-dependencies that may depend
on other elements.

For example, glib requires gobject and gdump generated files require
gmodule, so we've a cyclic dependency because gmodule requires gobject,
that requires glib.

To prevent this, let's just generate the introspection files at once in
a different meson file so that we don't have to deal with this.

As per this we could even revert commit fa37ab6d since gio is now
compiled before the gir files.