Use proposed go/types API

[bobg]

To support generics, modver duplicates some of the logic in go/types that is presently unexported. There are two proposals to export some of that logic:

• x/exp/typeparams: a new module with a transitional API for tools golang/go#50447
• proposal: go/types: export term/termlist logic golang/go#50963

When and if those proposals become adopted, modver should use those calls.