Use a more compact layers.idx format

[philwebb]

The current layers.idx format is quite verbose and also needs custom parsing code if third-parties are to read it.

We could probably improve the situation by writing layer details only once. We could also use folder names in the index if all sub-files are in the same layer.

[philwebb]

We're going to try a YAML compatible format that looks like this:

- "test":
  - "BOOT-INF/lib/a.jar"
  - "BOOT-INF/lib/b.jar"
- "application":
  - "BOOT-INF/classes/Demo.class"
  - "META-INF/"

Entries that end / mean everything in that folder is in the same layer.

The file above can be parsed in Go quite easily:

package main

import (
	"fmt"
	"gopkg.in/yaml.v2"
)

type Layer map[string][]string

func main() {
	data := `[{"deps": ["a", "b"]}, {"app": ["c", "d"]}]`
	layers := make([]Layer, 0)
	err := yaml.Unmarshal([]byte(data), &layers)
	if err != nil {
		panic(err)
	}
	for _, layer := range layers {
		for name, entries := range layer {
        		fmt.Println(name, "=>", entries)
    		}
	}
}