Configure subprojects in right order
Use an array instead of a dict to make sure we iterate over the list of subprojects in the right order, which is first GStreamer core, then gst-plugins-base, then other things.
Without this subprojects might get configured in random order, in which case gstreamer or gst-plugins-base libs might get picked up via pkg-config if they are also available installed, instead of being picked up from the subproject. This breaks all kinds of assumptions in gst-build and will likely have strange effects and/or lead to build failures such as
subprojects/gst-plugins-good/tests/check/meson.build:144:2: ERROR: 'gstreamer-plugins-base-1.0' is not a pkgconfig dependency