Get rid of PluginJetFilesProvider, use a more sane impl instead

idea/idea-test-framework/src/org/jetbrains/kotlin/idea/test/KotlinLightCodeInsightFixtureTestCase.kt

@@ -1,17 +1,6 @@
 /*
- * Copyright 2010-2017 JetBrains s.r.o.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
+ * Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
  */
 
 package org.jetbrains.kotlin.idea.test
@@ -32,13 +21,18 @@ import com.intellij.openapi.startup.StartupManager
 import com.intellij.openapi.util.io.FileUtil
 import com.intellij.openapi.util.text.StringUtil
 import com.intellij.openapi.vfs.newvfs.impl.VfsRootAccess
+import com.intellij.psi.PsiManager
+import com.intellij.psi.search.FileTypeIndex
+import com.intellij.psi.search.ProjectScope
 import com.intellij.testFramework.LightProjectDescriptor
 import com.intellij.testFramework.LoggedErrorProcessor
 import org.apache.log4j.Logger
 import org.jetbrains.kotlin.config.LanguageFeature
+import org.jetbrains.kotlin.idea.KotlinFileType
 import org.jetbrains.kotlin.idea.actions.internal.KotlinInternalMode
 import org.jetbrains.kotlin.idea.facet.configureFacet
 import org.jetbrains.kotlin.idea.facet.getOrCreateFacet
+import org.jetbrains.kotlin.psi.KtFile
 import org.jetbrains.kotlin.test.InTextDirectivesUtils
 import org.jetbrains.kotlin.test.KotlinTestUtils
 import org.jetbrains.kotlin.test.TestMetadata
@@ -193,3 +187,10 @@ fun configureLanguageVersion(fileText: String, project: Project, module: Module)
         }
     }
 }
+
+fun Project.allKotlinFiles(): List<KtFile> {
+    val virtualFiles = FileTypeIndex.getFiles(KotlinFileType.INSTANCE, ProjectScope.getProjectScope(this))
+    return virtualFiles
+        .map { PsiManager.getInstance(this).findFile(it) }
+        .filterIsInstance<KtFile>()
+}