Automatically detect bundled sources in plugin dependency

[artspb]

The Go plugin has a separate go-openapi.jar and ships it together with its sources. The plugin now has the following structure:

go
  | go-openapi.jar
  | src
    | go-openapi-src.jar

I know that some other plugins have the same structure. It'd be great if we can we make gradle-intellij-plugin attach sources automatically.

[zolotov]

relates to #207

[artspb]

Oh, nice. Might be even a duplicate.

[YannCebron]

I like this approach, as it would allow us to provide "open api" sources for any plugin used as dependency, including 3rd party. Without need of Marketplace to provide source artifacts separately. see #349