Apache Avro is not easily consumable from Composer. The small library is made available as a tarball download but is not packaged as a git repo that can easily be published to Packagist.
There are a few attempts at Composer/Packagist packages, but neither seem to be well supported or versioned.
@bd808 has a proof of concept at https://github.com/bd808/avro-php that extracted the lang/php section of https://github.com/apache/avro using git subtree and then added LICENSE.txt, NOTICE.txt, composer.json and README updates. This could be (a) moved to the wikimedia GitHub account or (b) imported into gerrit which would mirror to GitHub automagically for publication on Packagist.