Phillip Webb 7bc7d86ad4 Refine layer customization for Maven and Gradle
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
2020-04-03 00:03:21 -07:00
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