cmd/go: say to confirm import path when it's not found

Fixes #31366.

Change-Id: Ief26f53e7fe94bedb7db79d3d7130c4cdcec4281
Reviewed-on: https://go-review.googlesource.com/c/go/+/174179
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/modfetch/codehost/git.go

@@ -166,7 +166,7 @@ func (r *gitRepo) loadRefs() {
 	if err != nil {
 		if rerr, ok := err.(*RunError); ok {
 			if bytes.Contains(rerr.Stderr, []byte("fatal: could not read Username")) {
-				rerr.HelpText = "If this is a private repository, see https://golang.org/doc/faq#git_https for additional information."
+				rerr.HelpText = "Confirm the import path was entered correctly.\nIf this is a private repository, see https://golang.org/doc/faq#git_https for additional information."
 			}
 		}
 		r.refsErr = err

src/cmd/go/testdata/script/mod_get_private_vcs.txt

@@ -6,5 +6,6 @@ env GO111MODULE=on
 env GOPROXY=
 
 ! go get github.com/golang/nonexist
+stderr 'Confirm the import path was entered correctly.'
 stderr 'If this is a private repository, see https://golang.org/doc/faq#git_https for additional information.'
 ! stdout .