Configure subprojects in the right order

Configure subprojects in the 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