Provide a way to customize layers with the Gradle plugin

[mbhave]

[wilkinsona]

There's no easy and reliable way for us to map from BootJar's classpath (a FileCollection) to Maven coordinates. There's no guarantee that every jar on the classpath has come from a Maven repository and even for those that have, there's no guaranteed link between the FileCollection that represents the classpath and the Configuration(s) (from which we could get a dependency graph) that populated it.

The above is the same problem as we had with requires unpack which is why it's configured using a Spec<FileTreeElement> which is passed the FileCopyDetails for each jar file that's been added to the jar. I think we'll have to do something similar for layer customization with Gradle.