internal: add osutil as internal package to drop dependency on mewkiz/pkg

Code from mewkiz/pkg@518ade7.

Author: mewmew

internal/osutil/osutil.go

@@ -0,0 +1,20 @@
+// Package osutil implements OS utility functions.
+package osutil
+
+import (
+	"log"
+	"os"
+)
+
+// Exists reports whether the given file or directory exists.
+func Exists(path string) bool {
+	_, err := os.Stat(path)
+	if err == nil {
+		return true
+	}
+	if os.IsNotExist(err) {
+		return false
+	}
+	log.Printf("unable to stat path %q; %v", path, err)
+	return false
+}