Simplify layer customization logic for both Maven and Gradle and
refactor some internals of the Gradle plugin.
Both Maven and Gradle now use a simpler customization format that
consists of `application`, `dependencies` and `layer order` sections.
The `application`, `dependencies` configurations support one or more
`into` blocks that are used to select content for a specific layer.
Closes gh-20526
7bc7d86ad4