Fix return type for JavaInfo.getSourceJars()

getSourceJars() returns SkylarkList now.

Should be NestedSet as was before in JavaSkylarkApiProvider and to be consistent with other methods which returns collection of Jar files.

Also consider some cleaning in scope of this task, as this method can be already used by someone.

Start by locating JavaInfo.getSourceJars() and the related JavaSkylarkApiProvider API. Compare its current SkylarkList return type with the prior NestedSet type and the other collection-returning methods, then inspect existing uses before deciding what cleanup is in scope. Done means the API type is corrected and affected uses remain consistent.

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