Show warning when the adding package is not for command.

Now, `gex` can add packages which is not for command without any warnings.

```bash
# developers can add packages which is not for command
gex --add github.com/izumin5210/gex

# what developers really want to add
gex --add github.com/izumin5210/gex/cmd/gex
```

In this situation, developers cannot notice their mistake.
So, please implement to be able to show the warning.