Detect being inside of Ghost

[ErisDS]

With all the change around the commands for installing, building, deving & releasing Ghost, mistakes are bound to happen. E.g. today I tried to run ghost setup instead of gulp setup and got the following:

Command 'setup' not found! Type ghost help for usage.

Soon, we'll have a ghost setup command, so this could have done something weird!

At the moment ghost update is smart enough to output the following message:

Working directory is not a valid Ghost installation. Please run 'ghost update' again within a valid Ghost installation.

I think it would be a great enhancement to have ghost-cli tell me when I'm being dumb and trying to run a ghost command inside of ghost itself (e.g. a clone or a download). The simplest marker for this is if there is a package.json file with name ghost. If you find that, I probably didn't mean to run a ghost command.

In this case, it'd be good for all commands to output a message like:

ghost commands don't work inside of a clone or download, did you mean gulp?