meson: Cleanup and fix include files paths, using base path without repetitions

This was, in theory, part of !2991 (merged), but due to the gitlab slowdowns apaprently even if I had pushed it long time ago (as https://gitlab.gnome.org/GNOME/glib/-/tree/wip/add-built-headers-deps and before its approval), it went in without it, leaving main into a broken state 😡.

So basically ensures that generated sources are all created into includedir/glib-2.0/<subpath> and not just includedir/<subpath>.

/cc @xclaesse