Fix failure when running `dependencies` task when using layered mappings.

A SelfResolvingDependency has to be a FileCollectionDependency or a ModuleDependency by the looks of it.

src/main/java/net/fabricmc/loom/configuration/providers/mappings/LayeredMappingsDependency.java

@@ -37,8 +37,11 @@ import java.util.Map;
 import java.util.Objects;
 import java.util.Set;
 
+import org.gradle.api.Project;
 import org.gradle.api.artifacts.Dependency;
+import org.gradle.api.artifacts.FileCollectionDependency;
 import org.gradle.api.artifacts.SelfResolvingDependency;
+import org.gradle.api.file.FileCollection;
 import org.gradle.api.tasks.TaskDependency;
 import org.zeroturnaround.zip.ByteSource;
 import org.zeroturnaround.zip.ZipEntrySource;
@@ -53,15 +56,17 @@ import net.fabricmc.mappingio.adapter.MappingSourceNsSwitch;
 import net.fabricmc.mappingio.format.Tiny2Writer;
 import net.fabricmc.mappingio.tree.MemoryMappingTree;
 
-public class LayeredMappingsDependency implements SelfResolvingDependency {
+public class LayeredMappingsDependency implements SelfResolvingDependency, FileCollectionDependency {
 	private static final String GROUP = "loom";
 	private static final String MODULE = "mappings";
 
+	private final Project project;
 	private final MappingContext mappingContext;
 	private final LayeredMappingSpec layeredMappingSpec;
 	private final String version;
 
-	public LayeredMappingsDependency(MappingContext mappingContext, LayeredMappingSpec layeredMappingSpec, String version) {
+	public LayeredMappingsDependency(Project project, MappingContext mappingContext, LayeredMappingSpec layeredMappingSpec, String version) {
+		this.project = project;
 		this.mappingContext = mappingContext;
 		this.layeredMappingSpec = layeredMappingSpec;
 		this.version = version;
@@ -156,7 +161,7 @@ public class LayeredMappingsDependency implements SelfResolvingDependency {
 
 	@Override
 	public Dependency copy() {
-		return new LayeredMappingsDependency(mappingContext, layeredMappingSpec, version);
+		return new LayeredMappingsDependency(project, mappingContext, layeredMappingSpec, version);
 	}
 
 	@Override
@@ -167,4 +172,9 @@ public class LayeredMappingsDependency implements SelfResolvingDependency {
 	@Override
 	public void because(String s) {
 	}
+
+	@Override
+	public FileCollection getFiles() {
+		return project.files(resolve());
+	}
 }

src/main/java/net/fabricmc/loom/extension/LoomGradleExtensionApiImpl.java

@@ -121,7 +121,7 @@ public abstract class LoomGradleExtensionApiImpl implements LoomGradleExtensionA
 		LayeredMappingSpecBuilderImpl builder = new LayeredMappingSpecBuilderImpl();
 		action.execute(builder);
 		LayeredMappingSpec builtSpec = builder.build();
-		return new LayeredMappingsDependency(new GradleMappingContext(getProject(), builtSpec.getVersion().replace("+", "_").replace(".", "_")), builtSpec, builtSpec.getVersion());
+		return new LayeredMappingsDependency(getProject(), new GradleMappingContext(getProject(), builtSpec.getVersion().replace("+", "_").replace(".", "_")), builtSpec, builtSpec.getVersion());
 	}
 
 	@Override

src/test/groovy/net/fabricmc/loom/test/LoomTestConstants.groovy

@@ -26,7 +26,7 @@ package net.fabricmc.loom.test
 
 class LoomTestConstants {
     public final static String DEFAULT_GRADLE = "7.0.1"
-    public final static String PRE_RELEASE_GRADLE = "7.4-20210926222420+0000"
+    public final static String PRE_RELEASE_GRADLE = "7.4-20210927222644+0000"
 
     public final static String[] STANDARD_TEST_VERSIONS = [DEFAULT_GRADLE, PRE_RELEASE_GRADLE]
 }

src/test/groovy/net/fabricmc/loom/test/integration/MojangMappingsProjectTest.groovy

@@ -39,9 +39,11 @@ class MojangMappingsProjectTest extends Specification implements GradleProjectTe
 
 		when:
 			def result = gradle.run(task: "build")
+			def dependenciesResult = gradle.run(task: "dependencies")
 
 		then:
 			result.task(":build").outcome == SUCCESS
+			dependenciesResult.task(":dependencies").outcome == SUCCESS
 
 		where:
 			version << STANDARD_TEST_VERSIONS