Developers have already been using Composer's package.json to enable composer to install their packages. This has worked relatively well. There is the problem of end users without shell access, but I hope that tools like [[ https://github.com/CurosMJ/NoConsoleComposer | NoConsoleComposer ]] can address this.
The recent work on [[ https://www.mediawiki.org/wiki/Requests_for_comment/Extension_registration | extension registration ]] requires extensions to have another json file: extension.json. The problems this introduces have been discussed on [[ http://thread.gmane.org/gmane.science.linguistics.wikipedia.technical/81628 | wikitech-l ]].
Composer should be enhanced to understand the information that extension.json provides so that developers only have to maintain a single .json file.