improve plugins discovery performance

We are currently loading plugin commands stubs for every
command invocation to add support for Cobra v2 completion.
This cause a significant performance hit if there is a
lot of plugins in the user space (7 atm in Docker Desktop):

`docker --version` takes in current 23.0.1 ~93ms

Instead of removing completion for plugins to fix the
regression, we can slightly improve plugins discovery by
spawning a goroutine for each iteration in the loop when
listing plugins:

`docker --version` now takes ~38ms

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

cli-plugins/manager/manager.go

@@ -1,15 +1,18 @@
 package manager
 
 import (
+	"context"
 	"os"
 	"path/filepath"
 	"sort"
 	"strings"
+	"sync"
 
 	"github.com/docker/cli/cli/command"
 	"github.com/docker/cli/cli/config"
 	"github.com/fvbommel/sortorder"
 	"github.com/spf13/cobra"
+	"golang.org/x/sync/errgroup"
 	exec "golang.org/x/sys/execabs"
 )
 
@@ -146,19 +149,31 @@ func ListPlugins(dockerCli command.Cli, rootcmd *cobra.Command) ([]Plugin, error
 	}
 
 	var plugins []Plugin
+	var mu sync.Mutex
+	eg, _ := errgroup.WithContext(context.TODO())
 	for _, paths := range candidates {
-		if len(paths) == 0 {
-			continue
-		}
-		c := &candidate{paths[0]}
-		p, err := newPlugin(c, rootcmd)
-		if err != nil {
-			return nil, err
-		}
-		if !IsNotFound(p.Err) {
-			p.ShadowedPaths = paths[1:]
-			plugins = append(plugins, p)
-		}
+		func(paths []string) {
+			eg.Go(func() error {
+				if len(paths) == 0 {
+					return nil
+				}
+				c := &candidate{paths[0]}
+				p, err := newPlugin(c, rootcmd)
+				if err != nil {
+					return err
+				}
+				if !IsNotFound(p.Err) {
+					p.ShadowedPaths = paths[1:]
+					mu.Lock()
+					defer mu.Unlock()
+					plugins = append(plugins, p)
+				}
+				return nil
+			})
+		}(paths)
+	}
+	if err := eg.Wait(); err != nil {
+		return nil, err
 	}
 
 	sort.Slice(plugins, func(i, j int) bool {