Do a large chunk of the move to go

This commit moves a huge chunk of the code to go. The TODO.md document
indicates what is left to be done.

internal/execx/exec.go

@@ -0,0 +1,46 @@
+// Package execx defines extensions to the "os/exec" package
+package execx
+
+import (
+	"os/exec"
+
+	"github.com/tkw1536/goprogram/stream"
+)
+
+// ExecCommandError is returned by Exec when a command could not be executed.
+// This typically hints that the executable cannot be found, but may have other causes.
+const ExecCommandError = 127
+
+// Exec executes a system command with the specified input/output streams, working directory, and arguments.
+//
+// If the command executes, it's exit code will be returned.
+// If the command can not be executed, returns [ExecCommandError].
+func Exec(io stream.IOStream, workdir string, exe string, argv ...string) int {
+	// setup the command
+	cmd := exec.Command(exe, argv...)
+	cmd.Dir = workdir
+	cmd.Stdin = io.Stdin
+	cmd.Stdout = io.Stdout
+	cmd.Stderr = io.Stderr
+
+	// run it
+	err := cmd.Run()
+
+	// non-zero exit
+	if err, ok := err.(*exec.ExitError); ok {
+		return err.ExitCode()
+	}
+
+	// unknown error
+	if err != nil {
+		return ExecCommandError
+	}
+
+	// everything is fine!
+	return 0
+}
+
+// MustExec is like Exec, except that it returns true if the command exited successfully, and else false.
+func MustExec(io stream.IOStream, workdir string, exe string, argv ...string) bool {
+	return Exec(io, workdir, exe, argv...) == 0
+}