The update:lightning command should echo the final "composer require" command if composer.json is not writable

Problem/Motivation

The update:lightning command expects composer.json to be writable, but that might not be the case. If it isn't, then the command won't work, and users will be left high and dry.

We should implement a --no-write option which outputs the final set of Composer commands to run, and which defaults to TRUE if composer.json isn't writable.