// Copy testdata into GOPATH/src/testarchive, along with a go.mod file
	// declaring the same path.
	modRoot := filepath.Join(GOPATH, "src", "testcarchive")
	if err := cgotest.OverlayDir(modRoot, "testdata"); err != nil {
		log.Panic(err)
	}
	if err := os.Chdir(modRoot); err != nil {
		log.Panic(err)
	}
	os.Setenv("PWD", modRoot)
	if err := os.WriteFile("go.mod", []byte("module testcarchive\n"), 0666); err != nil {
		log.Panic(err)
	}

			}
			modroot := modload.PackageModRoot(ctx, r.path)
			if modroot == "" && str.HasPathPrefix(r.dir, cfg.GOROOTsrc) {
				modroot = cfg.GOROOTsrc
				gorootSrcCmd := filepath.Join(cfg.GOROOTsrc, "cmd")
				if str.HasPathPrefix(r.dir, gorootSrcCmd) {
					modroot = gorootSrcCmd
				}
			}
			if modroot != "" {
				if rp, err := modindex.GetPackage(modroot, r.dir); err == nil {