Remove drush.info (Remove sebastian/version in favor of composer-show --format=json)

[weitzman]

if we get rid of drush.info, sebastian/version can't get the version number since Drush has been installed via Composer. composer-show can.

So this isssue would make composer a required program which seems not good for production sites.

[weitzman]

Will be easier after #2788.

[weitzman]

That PR merged, but it isn't quite enough. Right now composerRoot/drupalRoot is only discovered during bootstrap phase 1. We need it very early during preflight.

[weitzman]

Actually, DrupalFinder is designed such that it doesn't find composerRoot unless it also finds drupalRoot. So this change would be problematic for the global Drush9 use case.