Add `ConfigurableFileCollection.finalizeValue()` to allow the value of the file collection to be finalized. The initial implementation is too eager, in that it calculates all of the files of the collection. Instead, it should calculate and finalize all of the roots of the collection. Later commits will improve this.
Replace 'Matchers' with 'CoreMatchers' So we do not require 'org.hamcrest:hamcrest-library' as additional dependency anymore. Which was only available for most of the tests because it leaked onto the test compile classpath.