[DO NOT MERGE] Import plugin using buildSrc instead of dependency

[pablisco]

Loads the plugin through buildSrc instead of using the dependency manually on the gradle script. Left old dependency for reference

[pablisco]

@pakoito I did this on chatkit: https://github.com/pusher/chatkit-android/blob/master/chatkit-core/build.gradle#L16 The gradle settings dynamically loads a separate folder as part of the project: https://github.com/pusher/chatkit-android/blob/master/settings.gradle#L5 It does cause some weirdenings on the project. The IDE gets confused about the dependencies and shows them when opening the imported project. Solved by doing a fresh import and IntelliJ though.