Add DSL to configure mod class path groups. (#608)

* Add basic mod settings.

* Cleanup and review feedback.

* Add idea output dir support.

* Eclipse and vscode + better tests.

src/main/java/net/fabricmc/loom/api/ModSettings.java

@@ -0,0 +1,55 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2022 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.api;
+
+import javax.inject.Inject;
+
+import org.gradle.api.Named;
+import org.gradle.api.provider.ListProperty;
+import org.gradle.api.tasks.SourceSet;
+import org.jetbrains.annotations.ApiStatus;
+
+/**
+ * A {@link Named} object for setting mod-related values. The {@linkplain Named#getName() name} should match the mod id.
+ */
+@ApiStatus.Experimental
+public abstract class ModSettings implements Named {
+	/**
+	 * List of classpath directories, used to populate the `fabric.classPathGroups` Fabric Loader system property.
+	 */
+	public abstract ListProperty<SourceSet> getModSourceSets();
+
+	@Inject
+	public ModSettings() {
+		getModSourceSets().finalizeValueOnRead();
+	}
+
+	/**
+	 * Mark a {@link SourceSet} output directories part of the named mod.
+	 */
+	public void sourceSet(SourceSet sourceSet) {
+		getModSourceSets().add(sourceSet);
+	}
+}