Use isolated project for accessing root project files

Author: inorichi

plugin-gradle/src/main/java/com/diffplug/gradle/spotless/BaseKotlinExtension.java

@@ -25,6 +25,8 @@
 
 import javax.annotation.Nullable;
 
+import org.gradle.api.file.Directory;
+
 import com.diffplug.common.collect.ImmutableList;
 import com.diffplug.common.collect.ImmutableSortedMap;
 import com.diffplug.spotless.FileSignature;
@@ -160,7 +162,9 @@ private KtlintConfig(
 				Map<String, Object> editorConfigOverride,
 				List<String> customRuleSets) throws IOException {
 			Objects.requireNonNull(version);
-			File defaultEditorConfig = new File(getProject().getRootDir(), ".editorconfig");
+			@SuppressWarnings("UnstableApiUsage")
+			Directory rootProjectDir = getProject().getIsolated().getRootProject().getProjectDirectory();
+			File defaultEditorConfig = rootProjectDir.file(".editorconfig").getAsFile();
 			FileSignature editorConfigPath = defaultEditorConfig.exists() ? FileSignature.signAsList(defaultEditorConfig) : null;
 			this.version = version;
 			this.editorConfigPath = editorConfigPath;