cmd/go/internal/modload: ignore $GOPATH/go.mod

The existing implementation returns a fatal error if $GOPATH/go.mod exists.
We couldn't figure out what directory the go tool was treating as $GOPATH
in the error message.

Fixes #46807

Change-Id: If9db4c0377f7c36af9c367398d3da494be04cd41
Reviewed-on: https://go-review.googlesource.com/c/go/+/539596
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>

src/cmd/go/internal/modload/init.go

@@ -485,7 +485,13 @@ func Init() {
 	if len(list) > 0 && list[0] != "" {
 		gopath = list[0]
 		if _, err := fsys.Stat(filepath.Join(gopath, "go.mod")); err == nil {
-			base.Fatalf("$GOPATH/go.mod exists but should not")
+			fmt.Fprintf(os.Stderr, "go: warning: ignoring go.mod in $GOPATH %v\n", gopath)
+			if RootMode == NeedRoot {
+				base.Fatal(ErrNoModRoot)
+			}
+			if !mustUseModules {
+				return
+			}
 		}
 	}
 }

src/cmd/go/testdata/script/env_issue46807.txt

@@ -0,0 +1,12 @@
+! go mod tidy
+stderr '^go: warning: ignoring go.mod in \$GOPATH'
+stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules'''
+
+go env
+stdout 'GOPATH='
+stderr '^go: warning: ignoring go.mod in \$GOPATH'
+
+-- $GOPATH/go.mod --
+module bug
+
+go 1.21