eventually there will be some component used by more than one extension. these dependencies should be managed centrally for wmf's mediawiki installation instead of putting everything into our own build.
this means still having WikidataBuildResources but not run composer install in it and maybe rename / have it be the "Wikidata" extension that we deploy.
the one problem with this is that we have some dependencies that are installed as extensions, in "$IP/extensions/...". It is weird and probably problematic to do that with composer, side-by-side with the other extensions that are handled as submodules. I am not sure where would put them instead or how we would handle this.
The extensions that currently get installed via composer in WikidataBuildResources need to then become submodules of the wmf branches of core.
Current Wikidata deployment process is documented at https://wikitech.wikimedia.org/wiki/How_to_deploy_Wikidata_code.