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

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 https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/230
see the discussion there for extra context.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3012>