Create a new layer for loader classes

Create a dedicated layer that is used to hold the launcher support
classes. The layer sits between `dependencies` and
`snapshot-dependencies` so that the layer is sensible for both
SNAPSHOT and RELEASE versions of Spring Boot

Closes gh-20529

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/test/java/org/springframework/boot/loader/tools/ImplicitLayerResolverTests.java

@@ -34,8 +34,8 @@ class ImplicitLayerResolverTests {
 
 	@Test
 	void iteratorReturnsLayers() {
-		assertThat(this.layers).containsExactly(StandardLayers.DEPENDENCIES, StandardLayers.SNAPSHOT_DEPENDENCIES,
-				StandardLayers.APPLICATION);
+		assertThat(this.layers).containsExactly(StandardLayers.DEPENDENCIES, StandardLayers.SPRING_BOOT_LOADER,
+				StandardLayers.SNAPSHOT_DEPENDENCIES, StandardLayers.APPLICATION);
 	}
 
 	@Test