A JSON configuration file is generated for core plugins, which maps
plugin names with sources to parse for docstrings.
The file is then opened by the configuration generator script, which
will now favor explicitly listed files to (usually wildcarded) paths
passed on its command line.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8231>
They are part of gst_dep already and we have to make sure to always have
gst_dep. The order in dependencies matters, because it is also the order
in which Meson will set -I args. We want gstreamer's config.h to take
precedence over glib's private config.h when it's a subproject.
While at it, remove useless fallback args for gmodule/gio dependencies,
only gstreamer core needs it.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2031>
