Requests for comment/Streamlining Composer usage

Background
MediaWiki core and its extensions depend on libraries that are managed via composer. This RFC intends to continue from where Requests for comment/Composer managed libraries for use on WMF cluster left off. Library infrastructure for MediaWiki will increase the use of Wikimedia maintained libraries hugely. To not hinder this effort we need to streamline the process for adding and upgrading composer dependencies and for building and deploying with composer.

Besides library dependencies composer can be used to build an autoloader for parts of core and parts of extensions. To properly make use of that we need to ensure that our build and deployment process works with that.