Show disabled commands with the reason why they were disabled #4035

The hooks from #3835 are now in, and a simple type of dependency resolution should now work.

However, I'd like to look further into providing feedback like in this mockup about why a command might not be available:

The before_add_command:<command> hook passes a WP_CLI\Dispatcher\CommandAddition object to the callback, which has an abort( $reason ) method.

I'm thinking about adding a new command object for this, something like WP_CLI\Dispatcher\DisabledCommand. When preparing the help screen, we can then still cycle over all commands, but provide different output for the disabled ones. They should also not be available to be actually run.

What do you think?