// be indexed because it contains symlinks.
func indexModule(modroot string) ([]byte, error) {
	fsys.Trace("indexModule", modroot)
	var packages []*rawPackage

	// If the root itself is a symlink to a directory,
	// we want to follow it (see https://go.dev/issue/50807).
	// Add a trailing separator to force that to happen.
	root := str.WithFilePathSeparator(modroot)

			walkPkgs(filepath.Join(cfg.GOROOTsrc, "cmd"), "cmd", pruneGoMod)
		}
	}

	if cfg.BuildMod == "vendor" {
		for _, mod := range MainModules.Versions() {
			if modRoot := MainModules.ModRoot(mod); modRoot != "" {
				walkPkgs(modRoot, MainModules.PathPrefix(mod), pruneGoMod|pruneVendor)
			}
		}
		if HasModRoot() {
			walkPkgs(VendorDir(), "", pruneVendor)
		}
		return
	}

		log.Fatal(err)
	}

	if os.Getenv("GO111MODULE") == "on" {
		out, err := exec.Command("go", "env", "GOMOD").Output()
		if err != nil {
			log.Fatal(err)
		}
		modRoot := filepath.Dir(strings.TrimSpace(string(out)))
		check(data, "module root", modRoot)
	} else {
		check(data, "GOPATH", os.Getenv("GOPATH"))
	}
	check(data, "GOROOT", os.Getenv("GOROOT"))
}

func check(data []byte, desc, dir string) {