IDEA-104500 Gradle: Allow to reuse common logic for other external systems

Don't check for build.gradle file existence at ide integration because it seems that gradle uses 'virtual projects' internally in case of multi-projects. Example: $GRADLE_HOME/samples/java/multiproject/settings.gradle defines 'include "shared", "api", "services:webservice", "services:shared"' and virtual 'services' project is created. Target directory doesn't contain build.gradle though

plugins/gradle/src/org/jetbrains/plugins/gradle/service/project/GradleExecutionHelper.java

@@ -133,9 +133,6 @@ public class GradleExecutionHelper {
     throws IllegalStateException
   {
     File projectFile = new File(projectPath);
-    if (!projectFile.isFile()) {
-      throw new IllegalArgumentException(GradleBundle.message("gradle.import.text.error.invalid.path", projectPath));
-    }
     File projectDir = projectFile.getParentFile();
     GradleConnector connector = GradleConnector.newConnector();
     if (settings != null) {