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

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-jar-layered-custom.gradle

@@ -9,27 +9,17 @@ bootJar {
 
 // tag::layered[]
 bootJar {
-	layers {
-		layersOrder "dependencies", "snapshot-dependencies", "application"
-		libraries {
-			layerContent("snapshot-dependencies") {
-				coordinates {
-					include "*:*:*SNAPSHOT"
-				}
-			}
-			layerContent("dependencies") {
-				coordinates {
-					include "*:*"
-				}
-			}
-		}
+	layered {
 		application {
-			layerContent("application") {
-				locations {
-					include "**"
-				}
-			}
+			intoLayer("application")
 		}
+		dependencies {
+			intoLayer("snapshot-dependencies") {
+				include "*:*:*SNAPSHOT"
+			}
+			intoLayer("dependencies")
+		}
+		layerOrder "dependencies", "snapshot-dependencies", "application"
 	}
 }
 // end::layered[]

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-jar-layered-custom.gradle.kts

@@ -7,27 +7,17 @@ plugins {
 
 // tag::layered[]
 tasks.getByName<BootJar>("bootJar") {
-	layers {
-		layersOrder("dependencies", "snapshot-dependencies", "application")
-		libraries {
-			layerContent("snapshot-dependencies") {
-				coordinates {
-					include("*:*:*SNAPSHOT")
-				}
-			}
-			layerContent("dependencies") {
-				coordinates {
-					include("*:*")
-				}
-			}
-		}
+	layered {
 		application {
-			layerContent("application") {
-				locations {
-					include("**")
-				}
-			}
+			intoLayer("application")
 		}
+		dependencies {
+			intoLayer("snapshot-dependencies") {
+				include("*:*:*SNAPSHOT")
+			}
+			intoLayer("dependencies") {
+		}
+		layersOrder("dependencies", "snapshot-dependencies", "application")
 	}
 }
 // end::layered[]

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-jar-layered.gradle

@@ -9,6 +9,6 @@ bootJar {
 
 // tag::layered[]
 bootJar {
-	layers()
+	layered()
 }
 // end::layered[]

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-jar-layered.gradle.kts

@@ -11,6 +11,6 @@ tasks.getByName<BootJar>("bootJar") {
 
 // tag::layered[]
 tasks.getByName<BootJar>("bootJar") {
-	layers()
+	layered()
 }
 // end::layered[]