Intead of passing around the output of the config generator program,
which consists of paths joined by a separator we can have the generator
simply produce an extra file containing those paths.
This commit only implements the new approach for the core plugins, as
this was needed to avoid spurious meson rebuilds when the pre-commit
hook regenerates the core plugins_cache.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8231>
