Add dummy task output for validate and format tasks so they are incremental (#155)

Author: haydenbaker

smithy-base/src/main/java/software/amazon/smithy/gradle/SmithyBasePlugin.java

@@ -158,6 +158,7 @@ private void addFormatTaskForSourceSet(SourceSet sourceSet, SmithySourceDirector
                 formatTask -> {
                     formatTask.getModels().set(sds.getSourceDirectories());
                     formatTask.setEnabled(extension.getFormat().get());
+                    formatTask.getOutputs().upToDateWhen(s -> true);
                 });
 
         // Smithy files should be formatted before they are built

smithy-jar/src/main/java/software/amazon/smithy/gradle/SmithyJarPlugin.java

@@ -145,6 +145,7 @@ private void addJavaTasksForSourceSet(SourceSet sourceSet, SmithyBuildTask build
 
                     // Add to verification group, so this tasks shows up in the output of `gradle tasks`
                     validateTask.setGroup(LifecycleBasePlugin.VERIFICATION_GROUP);
+                    validateTask.getOutputs().upToDateWhen(s -> true);
                 });
         project.getTasks().getByName("test").dependsOn(validateTaskProvider);
     }