Imports of txtar-x command

PathSynopsis
flagPackage flag implements command-line flag parsing.
fmtPackage fmt implements formatted I/O with functions analogous to C's printf and scanf.
github.com/rogpeppe/go-internal/txtarPackage txtar implements a trivial text-based file archive format.
ioPackage io provides basic interfaces to I/O primitives.
logPackage log implements a simple logging package.
osPackage os provides a platform-independent interface to operating system functionality.