Consider streamlining Composer updates

[yaronkoren]

With #463, Canasta went from calling "composer update" on all relevant extensions and skins (and core MediaWiki) at one time (from the file composer.local.json, which is copied from composer.canasta.json) to calling "composer update" for each such extension and skin (and core) separately, one at a time. This change made the code less hacky and, suprisingly, made it run faster. However, perhaps it makes sense to go back to a streamlined update, by adding lines for the relevant extensions and skins dynamically to the file composer.json, before it's called. This issue is here to discuss this potential change.