3.0 plugin bake

This fixes all problems created after adding the 'src' and 'tests' folder to plugins.

Now the PluginTask will also modify the app's composer.json file so that the plugin can be autoloaded

Fixes https://github.com/cakephp/cakephp/issues/3696
It could. Unfortunately it would not work the same for all tasks, that's why I decided to make it a bit more consistent across all of them by placing it in the bake function. Is it ok to change it only for those where it is possible?