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`.
@markstory suggested making `App::pluginPath()` return the path with `src/`. Earlier `App::pluginPath()` was just a wrapper to `Plugin::path()` and `src/` was appended to the path where needed.