Or perhaps a missile full of drones, similar to those bombs with lots of guided submunitions.

Tomahawk TLAM/D carries 166 submunitions in 24 canisters. There's no reason those submunitions need to be dumb — or stationary for that matter.

I can't seem to find any reference for what actual submunitions a Tomahawk carries but conventional cluster munitions can already be smart:

https://en.wikipedia.org/wiki/CBU-97_Sensor_Fuzed_Weapon

Indeed, cheap and available in massive amounts also makes very dangerous.

Funny that we see this in various computer games, ships with drone bays. What you mention would be a drone missile perhaps?

Or just a UGV following infantry.

https://youtu.be/hXNJhB41L-M

https://youtu.be/H_O85m6VXn8

And I suppose the carrier vehicle is somehow invulnerable to missiles? In fact, why wouldn't it just carry missiles in the first place?

A missile blows up in all directions, whereas a small drone can focus fire and concentrate energy on individuals. A swarm of tiny drones in a concentrated population area can potentially end up with a similar kill to mass ratio as early nuclear bombs. It is similar to the difference between a large conventional bomb and a cluster bomb but taken to another level.