Update Spring Boot layered jar support

[philwebb]

We found quite a few problems with the layered jar format so we changed direction quite late in the day. We now have a meta-data file instead of a an updated structure.

The fix #6 will probably need to be updated. The new format has now been documented here: https://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/htmlsingle/#executable-jar-war-index-files

[philwebb]

This Go code might also be useful. I used it to test that the layer.idx file can be loaded as YAML.

[nebhale]

Gotcha. It's going to be done over in the Paketo repo though.