Integrate symfony/console and split out composer plugin implementation

[icanhazstring]

We should remove the dependency to composer/composer altogether as there are some drawbacks to it:

• often conflicting with a direct dependency to composer/composer
• binary needs to have a cli application which is currently only working when invoking as composer plugin
• internal implementations relying real strict to composer dependencies

For the future, this should be done using symfony/console while maintaining a composer-plugin library as convenience implementation which will function the same way composer-unused does now.