func (rs *Requirements) GoVersion() string {
	v, _ := rs.rootSelected("go")
	if v == "" {
		panic("internal error: missing go version in modload.Requirements")
	}
	return v
}

// rootSelected returns the version of the root dependency with the given module
// path, or the zero module.Version and ok=false if the module is not a root
// dependency.
func (rs *Requirements) rootSelected(path string) (version string, ok bool) {

					// as a transitive dependency, that transitive dependency is not needed
					// by any package or test in the main module.
					_, ok := ld.requirements.rootSelected(m.Path)
					unused = !ok
				}
				if unused {
					fmt.Fprintf(os.Stderr, "unused %s\n", m.Path)
				}
			}
		}

		keep := keepSums(ctx, ld, ld.requirements, loadedZipSumsOnly)

			if keepPkgGoModSums {
				r := resolveReplacement(pkg.mod)
				keep[modkey(r)] = true
			}

			if rs.pruning == pruned && pkg.mod.Path != "" {
				if v, ok := rs.rootSelected(pkg.mod.Path); ok && v == pkg.mod.Version {
					// pkg was loaded from a root module, and because the main module has
					// a pruned module graph we do not check non-root modules for