Unexport system commands

This patch deprecates exported system commands and moves the
implementation details to an unexported function.

Commands that are affected include:

- system.NewVersionCommand
- system.NewInfoCommand
- system.NewSystemCommand
- system.NewEventsCommand
- system.NewInspectCommand

Signed-off-by: Alano Terblanche <18033717+Benehiko@users.noreply.github.com>

cli/command/system/version_test.go

@@ -20,7 +20,7 @@ func TestVersionWithoutServer(t *testing.T) {
 			return types.Version{}, errors.New("no server")
 		},
 	})
-	cmd := NewVersionCommand(cli)
+	cmd := newVersionCommand(cli)
 	cmd.SetArgs([]string{})
 	cmd.SetOut(cli.Err())
 	cmd.SetErr(io.Discard)