cmd/go/internal/modload: ensure that modRoot and targetPrefix are initialized in DirImportPath

For #37438

Change-Id: I2e1f47d567842ac5504b7b8ed0b3fba6f92d778b
Reviewed-on: https://go-review.googlesource.com/c/go/+/266340
Run-TryBot: Bryan C. Mills <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Trust: Bryan C. Mills <[email protected]>

Author: Bryan C. Mills

src/cmd/go/internal/modload/load.go

@@ -520,9 +520,10 @@ func ImportFromFiles(ctx context.Context, gofiles []string) {
 // DirImportPath returns the effective import path for dir,
 // provided it is within the main module, or else returns ".".
 func DirImportPath(dir string) string {
-	if modRoot == "" {
+	if !HasModRoot() {
 		return "."
 	}
+	LoadModFile(context.TODO())
 
 	if !filepath.IsAbs(dir) {
 		dir = filepath.Join(base.Cwd, dir)