Fix PluginFingerprint for plugin with dependencies

Fixes #1073

Author: tisoft

plugin-maven/src/main/java/com/diffplug/spotless/maven/incremental/PluginFingerprint.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2021 DiffPlug
+ * Copyright 2021-2022 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,9 +17,12 @@
 
 import java.io.IOException;
 import java.io.UncheckedIOException;
+import java.util.ArrayList;
 import java.util.Base64;
+import java.util.List;
 import java.util.Objects;
 
+import org.apache.maven.model.Dependency;
 import org.apache.maven.model.Plugin;
 import org.apache.maven.project.MavenProject;
 
@@ -77,6 +80,10 @@ public String toString() {
 	}
 
 	private static byte[] digest(Plugin plugin, Iterable<Formatter> formatters) {
+		List<Dependency> dependencies = plugin.getDependencies();
+		// dependencies can be an unserializable org.apache.maven.model.merge.ModelMerger$MergingList
+		// replace it with a serializable ArrayList
+		plugin.setDependencies(new ArrayList<>(dependencies));
 		try (ObjectDigestOutputStream out = ObjectDigestOutputStream.create()) {
 			out.writeObject(plugin);
 			for (Formatter formatter : formatters) {
@@ -86,6 +93,9 @@ private static byte[] digest(Plugin plugin, Iterable<Formatter> formatters) {
 			return out.digest();
 		} catch (IOException e) {
 			throw new UncheckedIOException("Unable to serialize plugin " + plugin, e);
+		} finally {
+			// reset the original list
+			plugin.setDependencies(dependencies);
 		}
 	}
 }