Change Details

As part of {T99084}, we will be re-using the composer LinkConstraint code, which requires adding a dependency upon composer/composer to our composer.json. Since composer doesn't really have stable releases yet, we'll pin to a specific commit and upgrade as necessary. For now I picked bc45d9185513575434021527d7756420e9f4b2cf (tree: https://github.com/composer/composer/tree/bc45d9185513575434021527d7756420e9f4b2cf) Before adding to mediawiki/vendor, this needs a security review, so assigned to Chris.