$ gohack get -vcs github.com/rogpeppe/go-internal/modfile
module "github.com/rogpeppe/go-internal/modfile" does not appear to be in use
all modules failed; not replacing anything

The module is github.com/rogpeppe/go-internal. But the intent of the command was clear: I want to look at the package github.com/rogpeppe/go-internal/modfile. gohack get should accept a package path and start hacking on the containing module.