Use a local Walk function

Because arduino-cli segfaults when sketchDir is a symbolic link, a simple
walk function is introduced with a twofold effect:
- fix the segfault
- allow to use symlinks

ref: https://www.google.com/search?q=golang+Walk+does+not+follow+symbolic+links

Author: d-a-v

arduino/builder/sketch.go

@@ -58,6 +58,33 @@ func SketchSaveItemCpp(item *sketch.Item, destPath string) error {
 	return nil
 }
 
+func SimpleLocalWalk(root string, walkFn func(path string, info os.FileInfo, err error) error) error {
+	info, err := os.Stat(root)
+
+	if err != nil {
+	    return walkFn(root, nil, err)
+	}
+
+	err = walkFn(root, info, err)
+	if err == filepath.SkipDir {
+	    return nil
+	}
+
+	if info.IsDir() {
+	    files, err := ioutil.ReadDir(root)
+        if err == nil {
+            for _, file := range files {
+                err = SimpleLocalWalk(root + string(os.PathSeparator) + file.Name(), walkFn)
+                if err == filepath.SkipDir {
+                    return nil
+                }
+            }
+        }
+    }
+
+	return nil
+}
+
 // SketchLoad collects all the files composing a sketch.
 // The parameter `sketchPath` holds a path pointing to a single sketch file or a sketch folder,
 // the path must be absolute.
@@ -86,7 +113,8 @@ func SketchLoad(sketchPath, buildPath string) (*sketch.Sketch, error) {
 
 	// collect all the sketch files
 	var files []string
-	err = filepath.Walk(sketchFolder, func(path string, info os.FileInfo, err error) error {
+	err = SimpleLocalWalk(sketchFolder, func(path string, info os.FileInfo, err error) error {
+
 		// ignore hidden files and skip hidden directories
 		if strings.HasPrefix(info.Name(), ".") {
 			if info.IsDir() {