Buildings with internal buffer should allow to be linked to import routes

After making this suggestion (https://ideas.captain-of-industry.com/suggestions/740347/vehicle-depto-should-allow-to-link-import-route-from-detpo-and-have-export-port)
I quickly realize if this was done for the other buildings with internal buffer, it would be way better than what we have to do today.
Basically buildings with internal buffers should allow to be connected to import routes, that way one can make sure they will be empty, and the material will only go where you want to, even without output belts.
From there you just hook to a ballancer and done.
Buildings that should allow to be selected as import routes:
- Shipyard
- Trading docks
- Vehicle depo
- any other building with internal buffer that is inaccesible from belts
Sure, one could just slap an output betl (two, one for each type) but imho being able to set import routes from another depo to those buildings would be far superior