Only generate models for local supertypes

Avoid generating models for classes
implementing external SPI (e.g. `FileFilter`).
Keep `toString` models intact as they're
commonly used as taint-propagation method
(e.g. see `Joiner`).

java/ql/test/utils/model-generator/p/AbstractImplOfExternalSPI.java

@@ -0,0 +1,13 @@
+package p;
+
+import java.io.File;
+import java.io.FileFilter;
+
+public abstract class AbstractImplOfExternalSPI implements FileFilter {
+
+    @Override
+    public boolean accept(File pathname) {
+        return false;
+    }
+
+}

java/ql/test/utils/model-generator/p/ImplOfExternalSPI.java

@@ -0,0 +1,19 @@
+package p;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+
+public class ImplOfExternalSPI extends AbstractImplOfExternalSPI {
+
+    @Override
+    public boolean accept(File pathname) {
+        try {
+            Files.createFile(pathname.toPath());
+        } catch (IOException e) {
+            e.printStackTrace();
+        }
+        return false;
+    }
+
+}

java/ql/test/utils/model-generator/p/MultipleImpls.java

@@ -1,9 +1,13 @@
 package p;
 
+import java.io.File;
+import java.io.FileFilter;
+import java.io.IOException;
+import java.nio.file.Files;
 import java.util.concurrent.Callable;
 
 public class MultipleImpls {
-    
+
     public static interface Strategy {
         String doSomething(String value);
     }
@@ -22,7 +26,7 @@ public class MultipleImpls {
             return null;
         }
 
-    }
+    }    
     public static class Strat2 implements Strategy {
         private String foo;