Can Go's `flag` package print usage?

Yes, you can do that by modifying flag.Usage:

var Usage = func() {
        fmt.Fprintf(os.Stderr, "Usage of %s:\n", os.Args[0])

        flag.PrintDefaults()
}

Usage prints to standard error a usage message documenting all defined command-line flags. The function is a variable that may be changed to point to a custom function.

Example use from outside of flag:

flag.Usage = func() {
    fmt.Fprintf(os.Stderr, "This is not helpful.\n")
}