ges: meson: use python.get_install_dir() to find the pygobject overrides dir

Instead of trying to hardcode site-packages paths for different platforms just use python.get_install_dir() from meson and let it deal with the rest. Also no longer try to import pygobject, which would otherwise not be required at build time.

python.get_install_dir() was at the beginning broken on Windows, but that was fixed in 0.60 via https://github.com/mesonbuild/meson/pull/9156 and since ges now requires >0.60 this can be ignored.

This change was motivated by the install path being wrong under MSYS2, where the unix install layout is used and the detection code not taking that into account.

This MR is a continuation of gst-editing-services!230 (closed) see the discussion there for extra context.