Imports of walk command

PathSynopsis
contextPackage context defines the Context type, which carries deadlines, cancellation signals, and other request-scoped values across API boundaries and between processes.
flagPackage flag implements command-line flag parsing.
github.com/tonistiigi/fsutil
osPackage os provides a platform-independent interface to operating system functionality.
stringsPackage strings implements simple functions to manipulate UTF-8 encoded strings.