cmd/go: disallow version string in go mod init module path

To prevent confusion, go mod init should not allow version strings in
the module path when provided as an argument. Instead, fail with a
useful error message.

Fixes #28803

Change-Id: I59272a91b042e32cef33c2e2116f760ca1def218
Reviewed-on: https://go-review.googlesource.com/c/150018
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modcmd/init.go

@@ -10,6 +10,7 @@ import (
 	"cmd/go/internal/base"
 	"cmd/go/internal/modload"
 	"os"
+	"strings"
 )
 
 var cmdInit = &base.Command{
@@ -37,5 +38,8 @@ func runInit(cmd *base.Command, args []string) {
 	if _, err := os.Stat("go.mod"); err == nil {
 		base.Fatalf("go mod init: go.mod already exists")
 	}
+	if strings.Contains(modload.CmdModModule, "@") {
+		base.Fatalf("go mod init: module path must not contain '@'")
+	}
 	modload.InitMod() // does all the hard work
 }