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
A bunch of tasks already override the getPath(). I think having that method do the right thing is important even if it requires a bit of extra code.