Fix missing builtBy() using Gradle 5

[mr-archano]

Problem

I quickly tried to apply SNAPSHOT-43 to an existing project using Gradle 5.x and I experienced the following error:

> No signature of method: org.gradle.api.internal.file.UnionFileTree.builtBy() is applicable for argument types: (org.gradle.api.internal.tasks.DefaultSourceSetOutput_Decorated) values: [main classes]
  Possible solutions: filter(groovy.lang.Closure), filter(org.gradle.api.specs.Spec), filter(org.gradle.api.specs.Spec), countBy(groovy.lang.Closure)

Solution

The tree used to accumulate the content of each dir in classesDirs using plus() is of type UnionFileTree, so does not contain builtBy() (while ConfigurableTree does). The simplest solution is to apply builtBy to each subtree instead of the cumulative one.