Add default implementation to the provider.

dataforge-context/src/commonMain/kotlin/hep/dataforge/context/AbstractPlugin.kt

@@ -30,8 +30,6 @@ abstract class AbstractPlugin(override val meta: Meta = Meta.EMPTY) : Plugin {
         dependencies.add(factory)
         return PluginDependencyDelegate(factory.type)
     }
-
-    override fun provideTop(target: String): Map<Name, Any> = emptyMap()
 }
 
 fun <T : Named> Collection<T>.toMap(): Map<Name, T> = associate { it.name to it }

dataforge-context/src/commonMain/kotlin/hep/dataforge/provider/Provider.kt

@@ -39,12 +39,10 @@ interface Provider {
      */
     val defaultChainTarget: String get() = ""
 
-
     /**
      * A map of direct children for specific target
      */
-    fun provideTop(target: String): Map<Name, Any> =
-        throw IllegalArgumentException("Target $target is not supported for $this")
+    fun provideTop(target: String): Map<Name, Any> = emptyMap()
 }
 
 fun Provider.provide(path: Path, targetOverride: String? = null): Any? {