Maintaing reference to Configuration in BaseGraalCompileTask breaks configuration caching

[nhoughto]

What happened?

Use of Gradle Configuration Caching fails because of an illegal type referenced in the task definition, some types are special and not allowed to be maintained in the configuration, like Configuration for the classpath variable. FileCollection should be used instead

* What went wrong:
Could not load the value of field `classpath` of task `:nativeImage` of type `com.palantir.gradle.graal.NativeImageTask`.
> Cannot set the value of a property of type org.gradle.api.artifacts.Configuration using a provider of type org.gradle.api.internal.file.DefaultFileCollectionFactory$ResolvingFileCollection.

What did you want to happen?

use FileCollection instead and configuration caching will work as expected:

Change

gradle-graal/src/main/java/com/palantir/gradle/graal/BaseGraalCompileTask.java

Line 53 in dfabe5f

private final Property<Configuration> classpath = getProject().getObjects().property(Configuration.class);

to:

-    private final Property<Configuration> classpath = getProject().getObjects().property(Configuration.class);
+    private final Property<FileCollection> classpath = getProject().getObjects().property(FileCollection.class);

and in Plugin

-            task.setClasspath(project.getConfigurations().named("runtimeClasspath"));
+            final JavaPluginExtension javaPlugin = project.getExtensions().getByType(JavaPluginExtension.class);
+            final SourceSetContainer sourceSets = javaPlugin.getSourceSets();
+            final FileCollection classpathFiles = sourceSets.getByName("main").getRuntimeClasspath();
+            task.setClasspath(project.provider(() -> classpathFiles));

this does change the types for the getters and setters, but is likely worth the impact?

Happy to PR it if open to it.