This is a placeholder ticket for the creation of subtickets and planning of killing the Wikidata build step. (@Addshore will do this soon)
During Wikimania @Legoktm and @Addshore took a first stab at killing the build and essentially discovered all of the harder items & items that need discussion before we can move forward.
[x] Review and classify all the composer dependencies as either libraries or extensions (https://etherpad.wikimedia.org/p/wikidatabuild)
[x] T173936 Ensure all "libraries" don't interact with MediaWiki and are independent
[x] T173942 Ensure all "extensions" are on Gerrit in mediawiki/extensions/...
[x] T173941 Move libraries to mediawiki-vendor (Checking that they do not contain any mediawiki related codes) and remove them from the build.
[x] T173940 Add extensions to make-wmf-branch script
[x] T176948 Move loading of extensions from build into mediawiki-config
[x] T174922 Decide what to do with the remaining JavaScript only stuff
[x] T177087 Make Wikibase-related JS libraries npm packages and include them as git submodules in Wikibase.git instead of installing as PHP libs using Composer
[x] T177060 Sort out the extension lists and localization stuff for deployment
[x] T175026 Deploy extensions as normal MW extensions (CommonSettings.php, etc.) and remove them from the build (note that we might need to set up manual classmaps for AutoloadClasses temporarily until {T99865} is resolved.
[x] T181708 Undeploy the Wikidata extension / the build!!!!!!!!
[ ] T181706 Remove the labs instance & project used for creating the daily builds & puppet role
[ ] T181707 Remove the WikidataBuilder user from gerrit groups
[ ] T181709 Cleanup Phabricator tickets relating to the build