There are some dependency also for extensions not strong required, but needed by tests to make unit tests or phan happy (https://gerrit.wikimedia.org/r/#/c/401407/).
It is possible that this extensions are not listed as "required". That means the old way is still needed or maybe a "suggest" key can help here to list them (T182472)

From a discussion I had to day, we would first need the ability to define an extension as being optional which is T182472

An example is AbuseFilter which can be used by itself. But it also has feature that enable themselves when CheckUser or CodeEditor extensions are present and we need CI to have those in order to run the integration tests between those extensions.

While Quibble learned to recursively process extension.json and automatically clone all the dependencies (via --resolve-requires), we can not define those optional dependencies yet in extension.json.

Note that in the context of CI, "dependencies" go both ways. See T318107: Zuul: Interpret extension dependencies as mutual.