Do not depend on all packages, suggest them instead. require-dev will still have them for testing.

also removed composer plugin and added it to requirements
this way the plugin is recognized by composer. would not work otherwise.

setting the repo type to yii2-extension also allows us to run composer
plugin and let it handle the extensions in this package

also removed superflous minimum-stability definition

not need to suggest packages in core. They are required by extension
composer.json anyway.