Sometimes the dev dependencies of Mail cause a conflict with other apps
or Nextcloud server. Most of the Symfony dev deps are pulled in via the
php-cs-fixer, while that binary is actually independent and it doesn't
need to load any of its classes into a Nextcloud process. Therefore we
can use the composer bin plugin to move the bin into its own directory.
`composer run cs:fix` works just like before.
Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>
