internal/imports: fix TestNoMainModule

https://golang.org/cl/179998 made it an error to run go mod download
without args and without a go.mod. Don't do that.

Fixes golang/go#32488

Change-Id: Icd845feb8fecae69b6a8363438bb9ee546a310dc
Reviewed-on: https://go-review.googlesource.com/c/tools/+/181298
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

internal/imports/mod_test.go

@@ -524,11 +524,16 @@ func setup(t *testing.T, main, wd string) *modTest {
 		WorkingDir:  filepath.Join(mainDir, wd),
 	}
 
-	// go mod tidy instead of download because tidy will notice dependencies
+	// go mod download gets mad if we don't have a go.mod, so make sure we do.
-	// in code, not just in go.mod files.
+	_, err = os.Stat(filepath.Join(mainDir, "go.mod"))
+	if err != nil && !os.IsNotExist(err) {
+		t.Fatalf("checking if go.mod exists: %v", err)
+	}
+	if err == nil {
 		if _, err := env.invokeGo("mod", "download"); err != nil {
 			t.Fatal(err)
 		}
+	}
 
 	return &modTest{
 		T:        t,