Simple heuristics for source jars

[advayDev1]

Auto deps works now with projects and Maven jars.
But if you use local file jars, it does not.

We should add a simple heuristic. If you include dir/library.jar in compile, DependencyConvertor creates dir/library-sources.jar if it exists in j2objcTranslation.

We should also add a j2objcConfig command: attachSources(nameOfExistingCompileDependency, nameOfSourceJarToUse). If attachSources is used, the heuristic is skipped and we just use the provided name.

[advayDev1]

The most common case for real users (sources on maven) is done.
In addition a method for doing what is specified here is implemented already, as long as you follow:
https://github.com/j2objc-contrib/j2objc-gradle/blob/master/dependencies.md#build-standalone-third-party-library