Use a more compact layers.idx format

Update the `layers.idx` format so that it is more compact and can be
parsed by third-parties as YAML.

Closes gh-20860

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/test/resources/org/springframework/boot/loader/tools/LayersIndexTests-.txt

@@ -0,0 +1,6 @@
+- "a"
+  - "aardvark"
+  - "cat"
+  - "dog"
+  - "hamster"
+  - "zerbra"

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/test/resources/org/springframework/boot/loader/tools/LayersIndexTests-writeToWhenAllFilesInFolderAreInNotInSameLayerUsesFiles.txt

@@ -0,0 +1,8 @@
+- "a":
+  - "a1/b1/c1"
+  - "a2/b1"
+- "b":
+  - "a1/b1/c2"
+  - "a2/b2"
+- "c":
+  - "a1/b2/"

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/test/resources/org/springframework/boot/loader/tools/LayersIndexTests-writeToWhenAllFilesInFolderAreInSameLayerUsesFolder.txt

@@ -0,0 +1,4 @@
+- "a":
+  - "a1/"
+- "b":
+  - "a2/"

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/test/resources/org/springframework/boot/loader/tools/LayersIndexTests-writeToWhenLayerNotUsedSkipsLayer.txt

@@ -0,0 +1,6 @@
+- "a":
+  - "a1"
+  - "a2"
+- "c":
+  - "c1"
+  - "c2"

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/test/resources/org/springframework/boot/loader/tools/LayersIndexTests-writeToWhenSimpleNamesSortsAlphabetically.txt

@@ -0,0 +1,6 @@
+- "a":
+  - "aardvark"
+  - "cat"
+  - "dog"
+  - "hamster"
+  - "zerbra"

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/test/resources/org/springframework/boot/loader/tools/LayersIndexTests-writeToWritesLayersInIteratorOrder.txt

@@ -0,0 +1,9 @@
+- "b":
+  - "b1"
+  - "b2"
+- "a":
+  - "a1"
+  - "a2"
+- "c":
+  - "c1"
+  - "c2"