make LayeredArchitecture immutable

For all properties except for `optionalLayers` we have followed the pattern of immutability, we should do the same here. Technically this is a breaking change, but since this immutability is the usual pattern that ArchUnit follows everywhere else, and the fluent API invites to simply chain these methods anyway, the risk is probably low.

Signed-off-by: Peter Gafert <peter.gafert@tngtech.com>
(cherry picked from commit 3e54892)

archunit/src/main/java/com/tngtech/archunit/library/Architectures.java

@@ -109,7 +109,7 @@ public static final class LayeredArchitecture implements ArchRule {
         private final Set<LayerDependencySpecification> dependencySpecifications;
         private final PredicateAggregator<Dependency> irrelevantDependenciesPredicate;
         private final Optional<String> overriddenDescription;
-        private boolean optionalLayers;
+        private final boolean optionalLayers;
 
         private LayeredArchitecture() {
             this(new LayerDefinitions(),
@@ -140,8 +140,7 @@ private LayeredArchitecture(LayerDefinitions layerDefinitions,
          */
         @PublicAPI(usage = ACCESS)
         public LayeredArchitecture withOptionalLayers(boolean optionalLayers) {
-            this.optionalLayers = optionalLayers;
-            return this;
+            return new LayeredArchitecture(layerDefinitions, dependencySpecifications, irrelevantDependenciesPredicate, overriddenDescription, optionalLayers);
         }
 
         private LayeredArchitecture addLayerDefinition(LayerDefinition definition) {