Check unused dependencies task can be up to date (#1426)

* Check unused dependencies task can be up to date

Declare check unused dependencies as having no outputs. This means that
it should be up to date provided that inputs have not changed.

* Remove erroneous mavenCentral()

* Failures

* fix

* Add generated changelog entries

changelog/@unreleased/pr-1426.v2.yml

@@ -0,0 +1,5 @@
+type: improvement
+improvement:
+  description: Check unused dependencies task can be up to date
+  links:
+  - https://github.com/palantir/gradle-baseline/pull/1426

gradle-baseline-java/src/main/groovy/com/palantir/baseline/tasks/CheckUnusedDependenciesTask.java

@@ -59,6 +59,7 @@ public CheckUnusedDependenciesTask() {
         sourceClasses = getProject().getObjects().property(FileCollection.class);
         ignore = getProject().getObjects().setProperty(String.class);
         ignore.set(Collections.emptySet());
+        getOutputs().upToDateWhen(_task -> true);
     }
 
     @TaskAction

gradle-baseline-java/src/test/groovy/com/palantir/baseline/BaselineExactDependenciesTest.groovy

@@ -232,6 +232,16 @@ class BaselineExactDependenciesTest extends AbstractPluginTest {
         result.task(':sub-project-no-deps:checkImplicitDependencies').getOutcome() == TaskOutcome.SUCCESS
     }
 
+    def 'check results can be up to date'() {
+        when:
+        setupMultiProject()
+        with(":sub-project-no-deps:checkUnusedDependencies").build();
+
+        then:
+        BuildResult result = with(":sub-project-no-deps:checkUnusedDependencies").build();
+        result.task(':sub-project-no-deps:checkUnusedDependencies').outcome == TaskOutcome.UP_TO_DATE
+    }
+
     def 'checkUnusedDependencies fails when a redundant project dep is present'() {
         when:
         setupMultiProject()