load plugin command stubs when required

We are currently loading plugin command stubs for every
invocation which still has a significant performance hit.
With this change we are doing this operation only if cobra
completion arg request is found.

- 20.10.23: `docker --version` takes ~15ms
- 23.0.1: `docker --version` takes ~93ms

With this change `docker --version` takes ~9ms

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>
(cherry picked from commit c39c711a18)

e2e/cli-plugins/run_test.go

@@ -83,7 +83,7 @@ func TestHelpBad(t *testing.T) {
 
 	res := icmd.RunCmd(run("help", "badmeta"))
 	res.Assert(t, icmd.Expected{
-		ExitCode: 0,
+		ExitCode: 1,
 		Out:      icmd.None,
 	})
 	golden.Assert(t, res.Stderr(), "docker-help-badmeta-err.golden")
@@ -110,8 +110,8 @@ func TestBadHelp(t *testing.T) {
 	res.Assert(t, icmd.Expected{
 		ExitCode: 0,
 		// This should be identical to the --help case above
-		Out: usage,
-		Err: shortHFlagDeprecated,
+		Out: shortHFlagDeprecated + usage,
+		Err: icmd.None,
 	})
 }