path/filepath: simplify EvalSymlinks for plan9

Plan 9 doesn't have symbolic links, so EvalSymlinks can be simplified
just to check validity of the path and call Clean. This saves a lot
of redundant file system activity.

Updates #50775

Change-Id: I84c24ef1d5e6e38fd19df2d37c72fbf883f0140d
Reviewed-on: https://go-review.googlesource.com/c/go/+/404954
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: David du Colombier <0intro@gmail.com>

src/path/filepath/symlink_plan9.go

@@ -0,0 +1,27 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package filepath
+
+import (
+	"os"
+	"strings"
+	"syscall"
+)
+
+func evalSymlinks(path string) (string, error) {
+	// Plan 9 doesn't have symbolic links, so no need for substitutions.
+	if len(path) > 0 {
+		// Check validity of path
+		_, err := os.Lstat(path)
+		if err != nil {
+			// Return the same error value as on other operating systems
+			if strings.HasSuffix(err.Error(), "not a directory") {
+				err = syscall.ENOTDIR
+			}
+			return "", err
+		}
+	}
+	return Clean(path), nil
+}

src/path/filepath/symlink_unix.go

@@ -1,4 +1,4 @@
-//go:build !windows
+//go:build !windows && !plan9
 
 package filepath