It's currently difficult to resolve dependency on eZ Publish legacy as it needs to be done by hand.
This should be handled by Composer to simplify a lot.
The dependency needs to be resolved for following repositories:

• ezp-next (mainly to have tests depending on legacy kernel running out of the box)
• ezpublish5 (optional, for easier installation)

In both cases, a custom installer will be probably needed to have a well configured legacy instance.