build: Make meson to look for gir dependencies at configure time
GNOME Shell dependencies are often not super-easy to track because they are loaded at runtime and there's no strict check anywhere.
In order to make easier to packagers and developers to figure out all the required (and not) runtime dependencies of gnome-shell, we can use meson to check how the system is currently configured, to make it clearer what are the shell expectations. This will even help when automated tests will be added to the shell.
Also list the optional dependencies in the summary.
Another option we could use, is to create a simple JS script that does the same, but that can be ran both at initialization (moving there some stuff from environment) and build time, but I thought that could be just a duplication, if we keep the dependencies in meson.
Deps have been gathered with grep "imports\.gi" --include "*.js" -rh js | grep -o "\b[A-Z][A-Za-z]\+\(,\|\s\|\b\)" | sed "s/,\?\s*$//g" | sort -u, not really scientific tool but should be enough