Add a `--classmap-authoritative (-a)` argument to `composer install`,
`composer update` and `composer dumpautoload`. This enables the same
authoritative classmap behavior as the existing `classmap-authoritative`
configuration setting. The option can be used for creating highly
optimized production autoloaders via `composer install --no-dev
--optimize-autoloader --classmap-authoritative` for projects where
multiple autoloaders are present and unnecessary `file_exists` calls
introduce performance issues.
Closes#4361
Bryan Davis