Sync plugin's folder structure with core.

Looking at [cakephp/acl](https://github.com/cakephp/acl) it felt icky to see class file mixed with other metadata files at root. With this change plugins will have 3 top level folders `src`, `tests` and `webroot`.
Could we make the src/ path segment part of what `App::pluginPath()` does? If possible I'd like to have apps and plugins look similar in the end.