Move the gokey cmd main to an importable package

[pdmccormick]

Thank you for this great utility! It is so handy in fact that I would love to be able to bundle it into other programs. To this end, this PR lifts the contents of cmd/gokey (a non-importable main package with an unexported main function) into cmd/gokey/gokeycmd (as an importable gokeycmd package with an exported Main function). This enables the gokey cmd main to be imported into other single executable binary, multiple use Go programs.

How exactly these bundled programs are invoked will vary, but the Busybox-style approach uses multiple symlinks pointing to a common executable, with the symlink name indicating which subprogram should be run.

For example, consider multicmd.go:

package main

import (
        "fmt"
        "os"
        "path/filepath"

        "github.com/cloudflare/gokey/cmd/gokey/gokeycmd"
)

func main() {
        switch filepath.Base(os.Args[0]) {
        case "helloworld":
                fmt.Println("Hello World!")
        case "gokey":
                gokeycmd.Main()
        default:
                fmt.Println("run me as an executable named either `helloworld` or `gokey`")
        }
}

If this program is compiled, and either the executable is renamed to gokey or helloworld, or symlinks with those names are created, the different sub-commands will be invoked accordingly.

$ go build multicmd.go
$ ln -s multicmd helloworld
$ ln -s multicmd gokey

$ ./helloworld
Hello World!
$ ./gokey -p gokey -r gokeycmd
1'amOFD4yF

Thank you for the consideration.

[ignatk]

Thanks for the contribution. FYI I have updated ci workflows and rebased the PR for the CI to run