Adding a new require-dev dependency to mw extensions passes the initial static analysis checks for this job, the dependencies are correctly installed and checked against. The second half of the job though, running the phpunit tests, uses vendor.git and only adds the require-dev dependencies from mw core and vendor, but not the extensions.
The current workaround is to add extension require-dev to vendor.git, but this doesn't seem very sustainable. quibble should pull in the appropriate require-dev parts from all installed extensions.
This is somewhere around quibble.cmd.QuibbleCmd.fetch_composer_dev