Transitive major-version upgrades

[bobg]

Seen in the wild (actual project names concealed to protect proprietary interests):

Module A exports function F that takes a *github.Client as an argument. Module B calls F.

Module A upgrades its go-github dependency from v47 to v48. Now F takes a *github.Client (a different type, because it's a different major version). Modver does not require a major-version upgrade in A, and B breaks.