`addDependency` should "name" it for better deduplicate resilience

[benjie]

If you have one step that does:

this.beforeDepId = this.addDependency($cursor)

and another of the same class that does:

this.afterDepId = this.addDependency($cursor)

then Grafast will see them as having the same dependencies even though one step will use this as the "before" and the other as the "after". This means that the step will have to specifically check this.beforeDepId === peer.beforeDepId && this.afterDepId === peer.afterDepId when performing deduplicate.

If we instead add names to dependencies then we can check the names line up before calling deduplicate, catching some obvious issues like this by knowing that they're not peers.