cmd/go: do not reject internal double-dots in path elements

The relative path element ".." is already rejected
by the checks for leading and trailing dots.

Fixes #27299

Change-Id: Ia8ab543c93288cdc0615abd6d22521d44bc56d72
Reviewed-on: https://go-review.googlesource.com/c/go/+/197720
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/get/path.go

@@ -44,9 +44,6 @@ func checkPath(path string, fileName bool) error {
 	if path[0] == '-' {
 		return fmt.Errorf("leading dash")
 	}
-	if strings.Contains(path, "..") {
-		return fmt.Errorf("double dot")
-	}
 	if strings.Contains(path, "//") {
 		return fmt.Errorf("double slash")
 	}

src/cmd/go/internal/module/module.go

@@ -231,9 +231,6 @@ func checkPath(path string, fileName bool) error {
 	if path[0] == '-' {
 		return fmt.Errorf("leading dash")
 	}
-	if strings.Contains(path, "..") {
-		return fmt.Errorf("double dot")
-	}
 	if strings.Contains(path, "//") {
 		return fmt.Errorf("double slash")
 	}

src/cmd/go/internal/module/module_test.go

@@ -80,7 +80,7 @@ var checkPathTests = []struct {
 	{"x./z", false, false, false},
 	{".x/z", false, false, true},
 	{"-x/z", false, false, false},
-	{"x..y/z", false, false, false},
+	{"x..y/z", true, true, true},
 	{"x.y/z/../../w", false, false, false},
 	{"x.y//z", false, false, false},
 	{"x.y/z//w", false, false, false},
@@ -173,6 +173,7 @@ var checkPathTests = []struct {
 	// When we do, we'll enable them everywhere, not just for GitHub.
 	{"github.com/user/unicode/испытание", false, false, true},
 
+	{".../x", false, false, false},
 	{"../x", false, false, false},
 	{"./y", false, false, false},
 	{"x:y", false, false, false},