allow referencing endorsed plugins

xtext-idea-gradle-plugin/src/main/java/org/xtext/gradle/idea/IdeaComponentPlugin.xtend

@@ -65,6 +65,9 @@ class IdeaComponentPlugin implements Plugin<Project> {
 			idea.pluginDependencies.externalDependencies.forEach [
 				assembleSandboxTask.rootSpec.addChild.into(id).from(idea.pluginsCache / id / version)
 			]
+			idea.pluginDependencies.endorsedDependencies.forEach [
+				assembleSandboxTask.rootSpec.addChild.into(id).from(idea.ideaHome / "plugins" / id)
+			]
 			val upstreamSandBoxTasks = idea.pluginDependencies.projectDependencies
 				.map[project.project(id)]
 				.map[(tasks.getAt(ASSEMBLE_SANDBOX_TASK_NAME) as AssembleSandbox)]

xtext-idea-gradle-plugin/src/main/java/org/xtext/gradle/idea/tasks/IdeaExtension.xtend

@@ -41,6 +41,10 @@ class IdeaExtension {
 				val unpackedDependencies = pluginDependencies.externalDependencies.map[
 					pluginsCache / id / version
 				]
+				+
+				pluginDependencies.endorsedDependencies.map[
+					getIdeaHome / "plugins"/ id
+				]
 				val dependencyClasses = unpackedDependencies
 					.map[project.files(it / "classes")]
 					.reduce[FileCollection a, FileCollection b| a.plus(b)]
@@ -111,6 +115,7 @@ class IdeaPluginRepositories implements Iterable<IdeaPluginRepository> {
 class IdeaPluginDependencies {
 
 	val dependencies = <IdeaPluginDependency>newTreeSet[$0.id.compareTo($1.id)]
+	val projectDependencies = <IdeaPluginDependency>newTreeSet[$0.id.compareTo($1.id)]
 
 	def IdeaPluginDependency id(String id) {
 		val dependency = new IdeaPluginDependency(id)
@@ -119,15 +124,21 @@ class IdeaPluginDependencies {
 	}
 
 	def project(String id) {
-		id(id)
+		val dependency = new IdeaPluginDependency(id)
+		projectDependencies += dependency
+		dependency
+	}
+
+	def getEndorsedDependencies() {
+		dependencies.filter[version == null]
 	}
 
 	def getExternalDependencies() {
 		dependencies.filter[version != null]
 	}
 
 	def getProjectDependencies() {
-		dependencies.filter[version == null]
+		projectDependencies
 	}
 }