// dstPath containing entries for the file mappings in packageFiles, as well
// as for the packages transitively imported by the package(s) in pkgs.
//
// pkgs may include any package pattern that is valid to pass to 'go list',
// so it may also be a list of Go source files all in the same directory.
func WriteImportcfg(t testing.TB, dstPath string, packageFiles map[string]string, pkgs ...string) {
	t.Helper()

	// filename must end with ".go"
	basename, ok := strings.CutSuffix(filepath.Base(filename), ".go")
	if !ok {
		t.Fatalf("filename doesn't end in .go: %s", filename)
	}
	objname := basename + ".o"
	outname := filepath.Join(outdirname, objname)

	importcfgfile := os.DevNull
	if len(packageFiles) > 0 || len(pkgImports) > 0 {