monitor: Move commands to a separated package

This commit moves monitor commands to `monior/commands` package.
Commands still need access to the `monitor` object and buildx controller so this
commit enables this via `Monitor` interface stored in `monitor/types`.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

monitor/commands/ps.go

@@ -0,0 +1,37 @@
+package commands
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"text/tabwriter"
+
+	"github.com/docker/buildx/monitor/types"
+)
+
+type PsCmd struct {
+	m      types.Monitor
+	stdout io.WriteCloser
+}
+
+func NewPsCmd(m types.Monitor, stdout io.WriteCloser) types.Command {
+	return &PsCmd{m, stdout}
+}
+
+func (cm *PsCmd) Info() types.CommandInfo {
+	return types.CommandInfo{HelpMessage: `list processes invoked by "exec". Use "attach" to attach IO to that process`}
+}
+
+func (cm *PsCmd) Exec(ctx context.Context, args []string) error {
+	plist, err := cm.m.ListProcesses(ctx)
+	if err != nil {
+		return err
+	}
+	tw := tabwriter.NewWriter(cm.stdout, 1, 8, 1, '\t', 0)
+	fmt.Fprintln(tw, "PID\tCURRENT_SESSION\tCOMMAND")
+	for _, p := range plist {
+		fmt.Fprintf(tw, "%-20s\t%v\t%v\n", p.ProcessID, p.ProcessID == cm.m.AttachedPID(), append(p.InvokeConfig.Entrypoint, p.InvokeConfig.Cmd...))
+	}
+	tw.Flush()
+	return nil
+}